A
A3 Paper size for documents measuring up to 11.7" x 17".
A4 Paper size for documents measuring up to 8.27" x 11.69".
A5 Paper size for documents measuring up to 5.83" x 8.28".
Absolute colorimetric A rendering intent that aims to maintain color accuracy at the expense of preserving relationships between colors, used to predict how images will appear when printed on paper or other substrate with a distinct color cast, such as newsprint. With absolute colorimetric rendering intent, colors that falls inside the destination gamut remain unchanged, while out-of gamut colors are clipped. colors are not scaled to the destination white point.
Absolute white In theory, a material that perfectly reflects all light energy at every visible wavelength. In practice, a solid white known spectral data used as the " reference white " for all measurements of absolute reflectance.
Absorption Occurs when light is partially or completely absorbed by a surface, converting its energy to heat.
Abstract In the photographic sense, an image that is conceived apart from concrete reality, generally emphasizing lines, colors and geometrical forms, and their relationship to one another.
Acid free Specially prepared mounting boards, album pages, tissues, and storage boxes designed to house prints for maximum archival permanence.
AD conversion Analog-Digital Conversion. In order to process a continuous analog signal in a computer (e.g. a photo), it must first be digitized, which means, converted into a specific mathematical format of binary code. Pictures are usually digitized with the help of a digital camera or a scanner.
Adobe RGB Due to technical limitations, monitors and printers are unable to reproduce all the colors we can see with our eyes, also called the "LAB" color space, symbolized by the horseshoe shape in the diagram below. The group of colors an average computer monitor can replicate is called the (additive) sRGB color space. The group of colors a printer can generate is called the (subtractive) CMYk color space. There are many types of CMYk, depending on the device. From the diagram you can see that certain colors are not visible on an average computer monitor but printable by a printer and vice versa. Higher-end digital cameras allow you to shoot in Adobe RGB (1998), which is larger than sRGB and CMYk. This will allow for prints with a wider range of colors. However, most monitors are only able to display colors within sRGB.
A4 Paper size for documents measuring up to 8.27" x 11.69".
A5 Paper size for documents measuring up to 5.83" x 8.28".
Absolute colorimetric A rendering intent that aims to maintain color accuracy at the expense of preserving relationships between colors, used to predict how images will appear when printed on paper or other substrate with a distinct color cast, such as newsprint. With absolute colorimetric rendering intent, colors that falls inside the destination gamut remain unchanged, while out-of gamut colors are clipped. colors are not scaled to the destination white point.
Absolute white In theory, a material that perfectly reflects all light energy at every visible wavelength. In practice, a solid white known spectral data used as the " reference white " for all measurements of absolute reflectance.
Absorption Occurs when light is partially or completely absorbed by a surface, converting its energy to heat.
Abstract In the photographic sense, an image that is conceived apart from concrete reality, generally emphasizing lines, colors and geometrical forms, and their relationship to one another.
Acid free Specially prepared mounting boards, album pages, tissues, and storage boxes designed to house prints for maximum archival permanence.
AD conversion Analog-Digital Conversion. In order to process a continuous analog signal in a computer (e.g. a photo), it must first be digitized, which means, converted into a specific mathematical format of binary code. Pictures are usually digitized with the help of a digital camera or a scanner.
Adobe RGB Due to technical limitations, monitors and printers are unable to reproduce all the colors we can see with our eyes, also called the "LAB" color space, symbolized by the horseshoe shape in the diagram below. The group of colors an average computer monitor can replicate is called the (additive) sRGB color space. The group of colors a printer can generate is called the (subtractive) CMYk color space. There are many types of CMYk, depending on the device. From the diagram you can see that certain colors are not visible on an average computer monitor but printable by a printer and vice versa. Higher-end digital cameras allow you to shoot in Adobe RGB (1998), which is larger than sRGB and CMYk. This will allow for prints with a wider range of colors. However, most monitors are only able to display colors within sRGB.
Advanced Photo System A new standard in consumer photography developed by Kodak and four other System Developing Companies - Canon, Fuji, Minolta and Nikon - based on a new film format and innovative film, camera and photofinishing technologies. Generally, APS cameras are more compact in size, weight and embodied most of the latest and most advance technologies available. There are options in various sizes of print out and it will even provide a thumbnail prints (Contact sheet) for you to select or preview prior to actual printing. There have a different series of lenses and some of the 35mm format AF lenses can even be shared (With limitation or effective focal length will increased). You can say, it is a different system camera all together. So much for the brighter side, but there are drawbacks as well and it is not that economical as I originally thought it supposed to be.
AF Abbreviation for " Autofocus ".
AF illumination lamp An infrared beam that the camera fires to assist in auto focusing.
AF sensor The sensor used to detect focus in cameras equipped with an autofocus function. Algorithm - a rule or process (often mathematical) used by a computer to make a decision or series of decisions.
Airbrush A digital tool which gives the effect of spraying a paint / air mixture over a image. Most graphic packages incorporate an assortment of brushes that are user definable.
Alpha The degree of opacity of an image in computer graphics. Not the same as luminance.
Ambient light The natural light in a scene.
Ambient temperature Crudely, room temperature. Some photographic chemicals can be used "at ambient" (with appropriate correction of processing times) while other types need to be heated or cooled to a specific temperature required by a particular process.
Anamorphic sizing Unequal scale change in the horizontal and vertical direction of a scanner. This enables the scanner to adjust the ratio in the horizontal and vertical direction.
Anti-aliasing Anti-aliasing makes the edges appear much smoother by averaging out the pixels around the edge. In this example some blue is added to the yellow edge pixels and some yellow is added to the blue edge pixels, thereby making the transition between the yellow circle and the blue background more gradual and smooth. Most image editing software packages have "anti-aliasing" options for typing fonts, drawing lines and shapes, making selections, etc. Anti-aliasing also occurs naturally in digital camera images and smoothens out the "jaggies". Read here about "anti-alias" filters.
Aliased: Anti-aliased
AF Abbreviation for " Autofocus ".
AF illumination lamp An infrared beam that the camera fires to assist in auto focusing.
AF sensor The sensor used to detect focus in cameras equipped with an autofocus function. Algorithm - a rule or process (often mathematical) used by a computer to make a decision or series of decisions.
Airbrush A digital tool which gives the effect of spraying a paint / air mixture over a image. Most graphic packages incorporate an assortment of brushes that are user definable.
Alpha The degree of opacity of an image in computer graphics. Not the same as luminance.
Ambient light The natural light in a scene.
Ambient temperature Crudely, room temperature. Some photographic chemicals can be used "at ambient" (with appropriate correction of processing times) while other types need to be heated or cooled to a specific temperature required by a particular process.
Anamorphic sizing Unequal scale change in the horizontal and vertical direction of a scanner. This enables the scanner to adjust the ratio in the horizontal and vertical direction.
Anti-aliasing Anti-aliasing makes the edges appear much smoother by averaging out the pixels around the edge. In this example some blue is added to the yellow edge pixels and some yellow is added to the blue edge pixels, thereby making the transition between the yellow circle and the blue background more gradual and smooth. Most image editing software packages have "anti-aliasing" options for typing fonts, drawing lines and shapes, making selections, etc. Anti-aliasing also occurs naturally in digital camera images and smoothens out the "jaggies". Read here about "anti-alias" filters.
Aliased: Anti-aliased
Aperture A small, circular opening inside the lens that can change in diameter to control the amount of light reaching the camera's sensor as a picture is taken. The aperture diameter is expressed in f-stops; the lower the number, the larger the aperture. For instance, the aperture opening when set to f/2.8 is larger than at f/8. The aperture and shutter speed together control the total amount of light reaching the sensor. A larger aperture passes more light through to the sensor. Many cameras have an aperture priority mode that allows you to adjust the aperture to your own liking. See also shutter speed.
Aperture priority An exposure mode on an automatic or autofocus camera that lets you set the aperture while the camera sets the shutter speed for proper exposure. If you change the aperture, or the light level changes, the shutter speed changes automatically. Apart from the sport or action arena, aperture priority is the most common & effective automatic preference in photography. It can also explained as automatic exposure system in which the lens aperture is set by the photographer, and the camera sets the shutter speed. Can be used in the stop-down mode with any lens that does not interfere with the metering system.
Aperture ring A ring, located on the outside of the lens usually behind the focusing ring, which is linked mechanically to the diaphragm to control the size of the aperture; it is engraved with a set of numbers called f-numbers or f-stops.
Application A computer program, such as an image editor or image browser.
Archival The ability of a material, including some printing papers and compact discs, to last for many years.
Archival Image An image meant to have lasting utility. Archival images are of a higher resolution and quality than the digital image to the user on-screen. The file format most often associated with archival images is TIFF, or Tagged Image File Format, as compared to on-screen viewing file format, which are usually JPEGs and GIFs.
Archival Processing Processing designed to protect a print or negative as much as possible from premature deterioration caused by chemical reactions.
Artifact Artifacts refer to a range of undesirable changes to a digital image caused by the sensor, optics, and internal image processing algorithms of the camera. The table below lists some of the common digital imaging artifacts.
Auto bracketing A system that automatically makes a series of exposures at different exposure settings when the shutter is released, to increase the chances of obtaining a correctly exposed image.
Auto exposure lock A push-button, switch, or lever that locks in exposure after the initial reading has been made, regardless of a change in camera position or light conditions after the lock is activated. Release of the lock button returns the exposure system to normal. Useful for making highlight or shadow readings of select portions of the frame, and an essential feature for critical exposure control with automated cameras.
Autofocus Device used in certain cameras, projectors and enlargers that focuses the image automatically.
Auto levels This is a very basic automatic correction technique employed by scanning and image processing software. Usually, the process takes an Integrated reading of the image and attempts to adjust the image by targeting a neutral scale for all values. This only works in a perfectly neutral image and often Auto Levels alone is insufficient to perform quality image optimization.
Aperture priority An exposure mode on an automatic or autofocus camera that lets you set the aperture while the camera sets the shutter speed for proper exposure. If you change the aperture, or the light level changes, the shutter speed changes automatically. Apart from the sport or action arena, aperture priority is the most common & effective automatic preference in photography. It can also explained as automatic exposure system in which the lens aperture is set by the photographer, and the camera sets the shutter speed. Can be used in the stop-down mode with any lens that does not interfere with the metering system.
Aperture ring A ring, located on the outside of the lens usually behind the focusing ring, which is linked mechanically to the diaphragm to control the size of the aperture; it is engraved with a set of numbers called f-numbers or f-stops.
Application A computer program, such as an image editor or image browser.
Archival The ability of a material, including some printing papers and compact discs, to last for many years.
Archival Image An image meant to have lasting utility. Archival images are of a higher resolution and quality than the digital image to the user on-screen. The file format most often associated with archival images is TIFF, or Tagged Image File Format, as compared to on-screen viewing file format, which are usually JPEGs and GIFs.
Archival Processing Processing designed to protect a print or negative as much as possible from premature deterioration caused by chemical reactions.
Artifact Artifacts refer to a range of undesirable changes to a digital image caused by the sensor, optics, and internal image processing algorithms of the camera. The table below lists some of the common digital imaging artifacts.
- Blooming - A pixel on a digital camera sensor collects photons which are converted into an electrical charge by its photodiode. As explained in the dynamic range topic, once the "bucket" is full, the charge caused by additional photons will overflow and have no effect on the pixel value, resulting in a clipped or overexposed pixel value. Blooming occurs when this charge flows over to surrounding pixels, brightening or overexposing them in the process. In the example below, the charge overflow of the overexposed pixels in the sky causes the dark pixels at the edges of the leaves and branches to be brightened and overexposed as well. As a result detail is lost. Blooming can also increase the visibility of purple fringing. Some sensors come with "anti-blooming gates" which drain away the overflowing charge so it does not affect the surrounding pixels, except for extreme exposures (very bright edge against a virtually black edge).
- Jaggles Hardly a technical term, jaggies refer to the visible "steps" of diagonal lines or edges in a digital image. Also referred to as "aliasing", these steps are simply a consequence of the regular, square layout of a pixel.
- JPEG Compression The most commonly used digital image format is JPEG (Joint Photographic Experts Group). Universally compatible with browsers, viewers, and image editing software, it allows photographic images to be compressed by a factor 10 to 20 compared to the uncompressed original with very little visible loss in image quality.
- Moire If a scene contains areas with repetitive detail which exceeds the resolution of the camera (1), a wavy moiré pattern (2) can appear, as shown in crop A. There is no moiré in crop B of an image of the same scene taken with a camera with a higher resolution. Anti-alias (3) filters reduce or eliminate moiré but also reduce image sharpness.
- Noise Each pixel in a camera sensor contains one or more light sensitive photodiodes which convert the incoming light (photons) into an electrical signal which is processed into the color value of the pixel in the final image. If the same pixel would be exposed several times by the same amount of light, the resulting color values would not be identical but have small statistical variations, called "noise". Even without incoming light, the electrical activity of the sensor itself will generate some signal, the equivalent of the background hiss of audio equipment which is switched on without playing any music. This additional signal is "noisy" because it varies per pixel (and over time) and increases with the temperature, and will add to the overall image noise. It is called the "noise floor". The output of a pixel has to be larger than the noise floor in order to be significant (i.e. to be distinguishable from noise).
- Sharpening Halo There are two types of sharpness and it is important not to mix them up. Optical sharpness is defined by the quality of the lens and the sensor. Software sharpness will create an "optical illusion" of sharpness by making the edges more contrasty. Software sharpening is of course unable to create detail beyond the camera's resolution, it will only help to bring out captured detail.
Auto bracketing A system that automatically makes a series of exposures at different exposure settings when the shutter is released, to increase the chances of obtaining a correctly exposed image.
Auto exposure lock A push-button, switch, or lever that locks in exposure after the initial reading has been made, regardless of a change in camera position or light conditions after the lock is activated. Release of the lock button returns the exposure system to normal. Useful for making highlight or shadow readings of select portions of the frame, and an essential feature for critical exposure control with automated cameras.
Autofocus Device used in certain cameras, projectors and enlargers that focuses the image automatically.
Auto levels This is a very basic automatic correction technique employed by scanning and image processing software. Usually, the process takes an Integrated reading of the image and attempts to adjust the image by targeting a neutral scale for all values. This only works in a perfectly neutral image and often Auto Levels alone is insufficient to perform quality image optimization.
B
Bleed Printing term referring to an image or inked area which extends to the edge of a printed piece. The bleed is the portion of the artwork that is beyond the trim marks of the piece. The bleed is required to account for any slight misalignment during trimming which would otherwise result in an unprinted strip of paper appearing at the edge of the finished piece.
Broad Lighting Conventional portraiture often positions the model in a pose such that their bodies are not facing the camera directly, but are turned to one side or the other. If the main light is positioned such that the illuminated portion of the face is framed in the image rather than the shadowed portion as in Short Lighting.
Buffer Memory in the camera that stores digital photos before they are written to the memory card.
Burning Selectively darkening part of a photo with an image editing program.
Broad Lighting Conventional portraiture often positions the model in a pose such that their bodies are not facing the camera directly, but are turned to one side or the other. If the main light is positioned such that the illuminated portion of the face is framed in the image rather than the shadowed portion as in Short Lighting.
Buffer Memory in the camera that stores digital photos before they are written to the memory card.
Burning Selectively darkening part of a photo with an image editing program.
C
CCD Charge Coupled Device: one of the two main types of image sensors used in digital cameras. When a picture is taken, the CCD is struck by light coming through the camera's lens. Each of the thousands or millions of tiny pixels that make up the CCD convert this light into electrons. The number of electrons, usually described as the pixel's accumulated charge, is measured, then converted to a digital value. This last step occurs outside the CCD, in a camera component called an analog-to-digital converter.
CD-R CD-Recordable: a compact disc that holds either 650 or 700 MB of digital information, including digital photos. Creating one is commonly referred to as burning a CD. A CD-R disc can only be written to once, and is an ideal storage medium for original digital photos.
CD-RW CD-Rewritable: similar in virtually all respects to a CD-R, except that a CD-RW disc can be written and erased many times. This makes them best suited to many backup tasks, but not for long term storage of original digital photos.
CMOS Complementary Metal-Oxide Semiconductor: one of the two main types of image sensors used in digital cameras. Its basic function is the same as that of a CCD. CMOS sensors are currently found in only a handful of digital cameras.
CMYK Cyan, Magenta, Yellow, Black. The four colors in the inksets of many photo-quality printers. Some printers use six ink colors to achieve smoother, more photographic prints. The two additional colors are often lighter shades of cyan and magenta.
Camera shake Movement of camera caused by unsteady hold or support, vibration, etc., leading, particularly at slower shutter speeds, to a blurred image on the film. It is a major cause of un-sharp pictures, especially with long focus lenses.
Channel Photoshop uses the term Channels to describes black and white and color image data. In Photoshop, one channel id typically defined as having up to eight bits of grayscale image information. Continuous tone images created with a scanner use channels as a way to describe the black and white and color image data. A black and white grayscale image has one channel. An RGB color image has three channels. A CMYK color image has four channels. It is possible to have up to 24 channels in photoshop.
Chroma The quality of a color that is the combination of hue and brightness. In the Munsell system of color notation, chroma indicates the purity of a color as measured along an axis; the farther from the axis, the purer the color.
CD-R CD-Recordable: a compact disc that holds either 650 or 700 MB of digital information, including digital photos. Creating one is commonly referred to as burning a CD. A CD-R disc can only be written to once, and is an ideal storage medium for original digital photos.
CD-RW CD-Rewritable: similar in virtually all respects to a CD-R, except that a CD-RW disc can be written and erased many times. This makes them best suited to many backup tasks, but not for long term storage of original digital photos.
CMOS Complementary Metal-Oxide Semiconductor: one of the two main types of image sensors used in digital cameras. Its basic function is the same as that of a CCD. CMOS sensors are currently found in only a handful of digital cameras.
CMYK Cyan, Magenta, Yellow, Black. The four colors in the inksets of many photo-quality printers. Some printers use six ink colors to achieve smoother, more photographic prints. The two additional colors are often lighter shades of cyan and magenta.
Camera shake Movement of camera caused by unsteady hold or support, vibration, etc., leading, particularly at slower shutter speeds, to a blurred image on the film. It is a major cause of un-sharp pictures, especially with long focus lenses.
Channel Photoshop uses the term Channels to describes black and white and color image data. In Photoshop, one channel id typically defined as having up to eight bits of grayscale image information. Continuous tone images created with a scanner use channels as a way to describe the black and white and color image data. A black and white grayscale image has one channel. An RGB color image has three channels. A CMYK color image has four channels. It is possible to have up to 24 channels in photoshop.
Chroma The quality of a color that is the combination of hue and brightness. In the Munsell system of color notation, chroma indicates the purity of a color as measured along an axis; the farther from the axis, the purer the color.
Chromatic aberration An optical defect of a lens which causes different colors or wave lengths of light to be focused at different distances from the lens. It is seen as color fringes or halos along edges and around every point in the image.
Color balance
( 1 ) The overall hue of the color in a color reproduction. Correct color balance implies that the colors in the scene are reproduced satisfactorily for the desired effect or realistic duplication.
( 2 ) Referring to color films, they are balanced in manufacture for exposure to light of a certain color quality ; i.e., daylight, tungsten, ect.
Color banding The appearance of visible bands of colors that replace subtle gradations in order to accommodate a reduced palette.
Color calibration The process of ensuring accurate reproduction of color for images. Full color calibration is usually a two-step process: calibrating your input device, such as a scanner; and calibrating your output device, such as a printer or monitor. By calibrating input and output devices correctly, color is accurately captured by your scanner and is reproduced faithfully on your monitor or printer as well.
Color cast The effect of one color dominating the overall look of an image. Often caused by improper exposure, wrong film type, or unusual lighting conditions when shooting the original image. Also caused, when scanning, by the sometimes unpredictable interaction between an image and a scanner.
Color channel Refers to the Red, Green and Blue components from which colors are created.
Color saturation The purity of a color resulting from the absence of black and white.
Color sensitivity Response of a sensitive material to colors of different wavelengths.
Color separation Conversion of RGB color information into its cyan, magenta, yellow and black constituents.
CompactFlash™ or Flash Drive A common type of digital camera memory card, about the size of a matchbook. There are two types of cards, Type I and Type II. They vary only in their thickness, with Type I being slightly thinner. A CompactFlash memory card can contain either flash memory or a miniature hard drive. The flash memory type is more prevalent.
Compression
Contrast The difference between the darkest and lightest areas in a photo. The greater the difference, the higher the contrast.
Crop (cropping) To trim the edges of an image, often to improve the composition. Cropping can be done by moving the camera position while viewing a scene, by adjusting the enlarger or easel during printing or by trimming the finished print.
Color balance
( 1 ) The overall hue of the color in a color reproduction. Correct color balance implies that the colors in the scene are reproduced satisfactorily for the desired effect or realistic duplication.
( 2 ) Referring to color films, they are balanced in manufacture for exposure to light of a certain color quality ; i.e., daylight, tungsten, ect.
Color banding The appearance of visible bands of colors that replace subtle gradations in order to accommodate a reduced palette.
Color calibration The process of ensuring accurate reproduction of color for images. Full color calibration is usually a two-step process: calibrating your input device, such as a scanner; and calibrating your output device, such as a printer or monitor. By calibrating input and output devices correctly, color is accurately captured by your scanner and is reproduced faithfully on your monitor or printer as well.
Color cast The effect of one color dominating the overall look of an image. Often caused by improper exposure, wrong film type, or unusual lighting conditions when shooting the original image. Also caused, when scanning, by the sometimes unpredictable interaction between an image and a scanner.
Color channel Refers to the Red, Green and Blue components from which colors are created.
Color saturation The purity of a color resulting from the absence of black and white.
Color sensitivity Response of a sensitive material to colors of different wavelengths.
Color separation Conversion of RGB color information into its cyan, magenta, yellow and black constituents.
CompactFlash™ or Flash Drive A common type of digital camera memory card, about the size of a matchbook. There are two types of cards, Type I and Type II. They vary only in their thickness, with Type I being slightly thinner. A CompactFlash memory card can contain either flash memory or a miniature hard drive. The flash memory type is more prevalent.
Compression
- JPEG A standard for compressing image data developed by the Joint Photographic Experts Group, hence the name JPEG. Strictly speaking, JPEG is not a file format, it's a compression method that is used within a file format, such as the EXIF-JPEG format common to digital cameras. It is referred to as a lossy format, which means some quality is lost in achieving JPEG's high compression rates. Usually, if a high-quality, low-compression JPEG setting is chosen on a digital camera, the loss of quality is not detectable to the eye.
- Lossless Compression Lossless compression is similar to what WinZip does. For instance, if you compress a document into a ZIP file and later extract and open the document, the content will of course be identical to the original. No information is lost in the process. Only some processing time was required to compress and decompress the document. TIFF is an image format that can be compressed in a lossless way.
- Lossy Compression Lossy compression reduces the image size by discarding information and is similar to summarizing a document. For example, you can summarize a 10 page document into a 9 page or 1 page document that represents the original, but you cannot create the original out of the summary as information was discarded during summarization. JPEG is an image format that is based on lossy compression.
Contrast The difference between the darkest and lightest areas in a photo. The greater the difference, the higher the contrast.
Crop (cropping) To trim the edges of an image, often to improve the composition. Cropping can be done by moving the camera position while viewing a scene, by adjusting the enlarger or easel during printing or by trimming the finished print.
D
Digital Zoom: To Use Or Not to Use Digital Zoom
So what is the best thing to do? If your purpose is to capture the information shown in image B, using a lens with focal length of 50mm is of course the best option. If you only have a 31mm lens available (or in general, if you reached the maximum optical zoom and need to zoom in more) there are three things you can do:
1. The recommended approach is to shoot image A with digital zoom OFF and crop it later the way you want it.
2. If the 5 megapixel camera has the option to output 2 megapixel images, then shoot with 1.6X digital zoom ON. The 1,600 x 1,200 crop will be saved without resampling and 2 megapixel of info is efficiently stored onto a 2 megapixel image. You save card space compared to image A, but lose the ability to change the way you cropped. This is recommended if card space is critical and is equivalent to cropping in the camera.
3. It is generally not recommended to shoot with 1.6X digital zoom ON and output it as a 5 megapixel image because you are combining the disadvantages of 1. (more card space) and 2. (lose cropping flexibility) without major benefits*. You are saving 2 megapixel of information (crop C) into a 5 megapixel upsampled image (D). Upsampling cannot create detail that was not captured by the lens. Image B (optical zoom) has more detail than image D (digital zoom).
* If for some reason your intention is to upsample and you are shooting in JPEG, one benefit of digital zoom is that the upsampling in the camera is done before JPEG compression. If you shoot A, crop the 1,600 x 1,200 area, and then upsample to 2,560 x 1,920 on your computer, you will magnify the JPEG compression artifacts and the upsampled image will look not as good as image D. Because not all digital zooms are created equally, you may want to verify the quality differences with your particular digital camera before using digital zoom for this purpose.
Digital camera A camera that captures the photo not on film, but in an electronic imaging sensor that takes the place of film.
Dodging Selectively lightening part of a photo with an image editing program.
DPI Dots per inch: A measurement of the resolution of a digital photo or digital device, including digital cameras and printers. The higher the number, the greater the resolution.
So what is the best thing to do? If your purpose is to capture the information shown in image B, using a lens with focal length of 50mm is of course the best option. If you only have a 31mm lens available (or in general, if you reached the maximum optical zoom and need to zoom in more) there are three things you can do:
1. The recommended approach is to shoot image A with digital zoom OFF and crop it later the way you want it.
2. If the 5 megapixel camera has the option to output 2 megapixel images, then shoot with 1.6X digital zoom ON. The 1,600 x 1,200 crop will be saved without resampling and 2 megapixel of info is efficiently stored onto a 2 megapixel image. You save card space compared to image A, but lose the ability to change the way you cropped. This is recommended if card space is critical and is equivalent to cropping in the camera.
3. It is generally not recommended to shoot with 1.6X digital zoom ON and output it as a 5 megapixel image because you are combining the disadvantages of 1. (more card space) and 2. (lose cropping flexibility) without major benefits*. You are saving 2 megapixel of information (crop C) into a 5 megapixel upsampled image (D). Upsampling cannot create detail that was not captured by the lens. Image B (optical zoom) has more detail than image D (digital zoom).
* If for some reason your intention is to upsample and you are shooting in JPEG, one benefit of digital zoom is that the upsampling in the camera is done before JPEG compression. If you shoot A, crop the 1,600 x 1,200 area, and then upsample to 2,560 x 1,920 on your computer, you will magnify the JPEG compression artifacts and the upsampled image will look not as good as image D. Because not all digital zooms are created equally, you may want to verify the quality differences with your particular digital camera before using digital zoom for this purpose.
Digital camera A camera that captures the photo not on film, but in an electronic imaging sensor that takes the place of film.
Dodging Selectively lightening part of a photo with an image editing program.
DPI Dots per inch: A measurement of the resolution of a digital photo or digital device, including digital cameras and printers. The higher the number, the greater the resolution.
E
EXIF Exchangeable Image File: the file format used by most digital cameras. For example, when a typical camera is set to record a JPEG, it's actually recording an EXIF file that uses JPEG compression to compress the photo data within the file.
External flash A supplementary flash unit that connects to the camera with a cable, or is triggered by the light from the camera's internal flash. Many fun and creative effects can be created with external flash.
External flash A supplementary flash unit that connects to the camera with a cable, or is triggered by the light from the camera's internal flash. Many fun and creative effects can be created with external flash.
F
Fill flash A flash technique used to brighten deep shadow areas, typically outdoors on sunny days. Some digital cameras include a fill flash mode that forces the flash to fire, even in bright light.
FireWire A type of cabling technology for transferring data to and from digital devices at high speed. Some professional digital cameras and memory card readers connect to the computer over FireWire. FireWire card readers are typically faster than those that connect via USB. Also known as IEEE 1394, FireWire was invented by Apple Computer but is now commonly used with Windows-based PCs as well.
FireWire A type of cabling technology for transferring data to and from digital devices at high speed. Some professional digital cameras and memory card readers connect to the computer over FireWire. FireWire card readers are typically faster than those that connect via USB. Also known as IEEE 1394, FireWire was invented by Apple Computer but is now commonly used with Windows-based PCs as well.
G
Gamma Each pixel in a digital image has a certain level of brightness ranging from black (0) to white (1). These pixel values serve as the input for your computer monitor. Due to technical limitations, CRT monitors output these values in a nonlinear way:
Output = Input ^ Gamma
When unadjusted, most CRT monitors have a "gamma" of 2.5 which means that pixels with a brightness of 0.5, will be displayed with a brightness of only 0.18 (0.5^2.5) in non-colormanaged applications (*). LCDs, in particular those on notebooks, tend to have rather irregularly shaped output curves. Calibration via software and/or hardware ensures that the monitor outputs the image based on a predetermined gamma curve, typically 2.2 for Windows, which is approximately the inverse of the response of the human vision. The sRGB and Adobe RGB color spaces are also based on a gamma of 2.2.
Output = Input ^ Gamma
When unadjusted, most CRT monitors have a "gamma" of 2.5 which means that pixels with a brightness of 0.5, will be displayed with a brightness of only 0.18 (0.5^2.5) in non-colormanaged applications (*). LCDs, in particular those on notebooks, tend to have rather irregularly shaped output curves. Calibration via software and/or hardware ensures that the monitor outputs the image based on a predetermined gamma curve, typically 2.2 for Windows, which is approximately the inverse of the response of the human vision. The sRGB and Adobe RGB color spaces are also based on a gamma of 2.2.
H
Highlights The brightest parts of a photo.
Histogram Histograms are the key to understanding digital images. This 10x4 mosaic contains 40 tiles which we could sort by color and then stack up accordingly. The higher the pile, the more tiles of that color in the mosaic. The resulting "histogram" would represent the color distribution of the mosaic.
Histogram Histograms are the key to understanding digital images. This 10x4 mosaic contains 40 tiles which we could sort by color and then stack up accordingly. The higher the pile, the more tiles of that color in the mosaic. The resulting "histogram" would represent the color distribution of the mosaic.
In the sensor topic we learned that a digital image is basically a mosaic of square tiles or "pixels" of uniform color which are so tiny that it appears uniform and smooth. Instead of sorting them by color, we could sort these pixels into 256 levels of brightness from black (value 0) to white (value 255) with 254 gray levels in between. Just as we did manually for the mosaic, an imaging software automatically sorted the pixels of the image below into 256 groups (levels) of "brightness" and stacked them up accordingly. The height of each "stack" or vertical "bar" tells you how many pixels there are for that particular brightness. "0" and "255" are the darkest and brightest values, corresponding to black and white respectively.
On this histogram each "stack" or "bar" is one pixel wide. Unlike the mosaic histograms, the 256 bars are stacked side by side without any space between them, which is why for educational purposes, the vertical bars are shown in alternating shades of gray, allowing you to distinguish the individual bars. There are no blank spaces between bars to avoid confusion with blank spaces caused by missing tones in the image. Normally all bars will be black as indicated in the second histogram.
I
Interpolation Interpolation (sometimes called resampling) is an imaging method to increase (or decrease) the number of pixels in a digital image. Some digital cameras use interpolation to produce a larger image than the sensor captured or to create digital zoom. Virtually all image editing software support one or more methods of interpolation. How smoothly images are enlarged without introducing jaggies depends on the sophistication of the algorithm.
L
LCD Liquid Crystal Display: a low-power monitor often used on the top and/or rear of a digital camera to display settings or the photo itself.
M
Media Material that information is written to and stored on. Digital photography storage media includes CompactFlash cards and CDs.
P
Pixel Picture Element: digital photographs are comprised of thousands or millions of them; they are the building blocks of a digital photo.
Posterization When performing image manipulations such as tonal conversions in low bit environments (e.g. 8bits/channel mode) only a limited number of tones may be available to describe a certain area of the image, as shown in this exaggerated example. This causes visible "banding" or "posterization".
Posterization When performing image manipulations such as tonal conversions in low bit environments (e.g. 8bits/channel mode) only a limited number of tones may be available to describe a certain area of the image, as shown in this exaggerated example. This causes visible "banding" or "posterization".
R
RAW Unlike JPEG and TIFF, RAW is not an abbreviation but literally means "raw" as in "unprocessed". A RAW file contains the original image information as it comes off the sensor before in-camera processing so you can do that processing afterwards on your PC with special software. Even though the TIFF file only retains 8 bits/channel of information, it will take up twice the storage space because it has three 8 bit color channels versus one 12 bit RAW channel. JPEG addresses this issue by compression, at the cost of image quality. So RAW offers the best of both worlds as it preserves the original color bit depth and image quality and saves storage space compared to TIFF. Some cameras offer nearly lossless compressed RAW.
The Flexibility of RAW
In addition, many of the camera settings which were applied to the raw data can be undone when using the RAW processing software. For instance, sharpening, white balance, levels and color adjustments can be undone and recalculated based on the raw data. Also, because RAW has 12 bits of available data, you are able to extract shadow and highlight detail which would have been lost in the 8 bits/channel JPEG or TIFF format.
Disadvantages of RAW
The only drawback is that RAW formats differ between camera manufacturers, and even between cameras, so dedicated software provided by the manufacturer has to be used. Furthermore, opening and processing RAW files is much slower than JPEG or TIFF files. To address this issue, some cameras are offering the option to shoot in RAW and JPEG at the same time. As cameras become faster and memory cards cheaper, this option has no longer performance or storage issues. It allows you to organize and edit your images in a faster way with regular software using the JPEGs. But you retain the option to process in RAW those critical images or images with problems (e.g. white balance or lost shadow and highlight detail). Another trend is that third party image editing and viewing software packages are becoming RAW compatible with most popular camera brands and models. An example is Adobe Photoshop CS. However, as stated in my Photoshop CS review, the way Photoshop processes RAW files can be different from the way the camera manufacturer's software does it and not all settings may be recognized.
Red-eye The red glow from a subject's eyes caused by light from a flash reflecting off the blood vessels behind the retina in the eye. The effect is most common when light levels are low, outdoor at night, or indoor in a dimly-lit room.
RGB Red, Green, Blue: the three colors to which the human visual system, digital cameras and many other devices are sensitive.
The Flexibility of RAW
In addition, many of the camera settings which were applied to the raw data can be undone when using the RAW processing software. For instance, sharpening, white balance, levels and color adjustments can be undone and recalculated based on the raw data. Also, because RAW has 12 bits of available data, you are able to extract shadow and highlight detail which would have been lost in the 8 bits/channel JPEG or TIFF format.
Disadvantages of RAW
The only drawback is that RAW formats differ between camera manufacturers, and even between cameras, so dedicated software provided by the manufacturer has to be used. Furthermore, opening and processing RAW files is much slower than JPEG or TIFF files. To address this issue, some cameras are offering the option to shoot in RAW and JPEG at the same time. As cameras become faster and memory cards cheaper, this option has no longer performance or storage issues. It allows you to organize and edit your images in a faster way with regular software using the JPEGs. But you retain the option to process in RAW those critical images or images with problems (e.g. white balance or lost shadow and highlight detail). Another trend is that third party image editing and viewing software packages are becoming RAW compatible with most popular camera brands and models. An example is Adobe Photoshop CS. However, as stated in my Photoshop CS review, the way Photoshop processes RAW files can be different from the way the camera manufacturer's software does it and not all settings may be recognized.
Red-eye The red glow from a subject's eyes caused by light from a flash reflecting off the blood vessels behind the retina in the eye. The effect is most common when light levels are low, outdoor at night, or indoor in a dimly-lit room.
RGB Red, Green, Blue: the three colors to which the human visual system, digital cameras and many other devices are sensitive.
S
Sensor Resolution The number of effective non-interpolated pixels on a sensor is discussed in the topic about pixels.
Image Resolution
The resolution of a digital image is defined as the number of pixels it contains. A 5 megapixel image is typically 2,560 pixels wide and 1,920 pixels high and has a resolution of 4,915,200 pixels, rounded off to 5 million pixels. It is recommended to shoot at a resolution which corresponds with the camera's effective pixel count. As explained in the pixels topic, shooting at higher (interpolated) resolutions (if available as an option) creates only marginal benefits but takes up more card space. Shooting at lower resolutions only makes sense if you are running out of card space and/or image quality is not important.
Resolution Charts at dpreview.com: Horizontal and Vertical LPH
We measure resolution using the widely accepted PIMA/ISO 12233 camera resolution test chart. This chart is excellent, not only for measuring pure horizontal and vertical resolution, but also to test the performance of the sensor with frequencies at various angles. It also offers a good reference point for comparison of resolution between cameras. The chart is available for every camera which comes through our test labs, both in the camera reviews and our extensive camera database.
Saturation How rich the colors are in a photo.
Sensitivity (ISO) Conventional film comes in different sensitivities (ASAs) for different purposes. The lower the sensitivity, the finer the grain, but more light is needed. This is excellent for outdoor photography, but for low-light conditions or action photography (where fast shutterspeeds are needed), more sensitive or "fast" film is used which is more "grainy".
Likewise, digital cameras have an ISO rating indicating their level of sensitivity to light. ISO 100 is the "normal" setting for most cameras, although some go as low as ISO 50. The sensitivities can be increased to 200, 400, 800, or even 3,200 on high-end digital SLRs. When increasing the sensitivity, the output of the sensor is amplified, so less light is needed. Unfortunately that also amplifies the undesired noise. Incidentally, this creates more grainy pictures, just like in conventional photography, but because of different reasons. It is similar to turning up the volume of a radio with poor reception. Doing so will not only amplify the (desired) music but also the (undesired) hiss and crackle or "noise". Improvements in sensor technology are steadily reducing the noise levels at higher ISOs, especially on higher-end cameras. And unlike conventional film cameras which require a change of film roll or the use of multiple bodies, digital cameras allow you to instantly and conveniently change the sensitivity depending on the circumstances.
Sharpening There are two types of sharpness and it is important not to mix them up. Optical sharpness is defined by the quality of the lens and the sensor. Software sharpness will create an "optical illusion" of sharpness by making the edges more contrasty. Software sharpening is of course unable to create detail beyond the camera's resolution, it will only help to bring out captured detail.
In-camera Sharpening
Digital cameras will, as a part of their default image processing, apply some level of sharpening, to counteract the effects of the interpolation of colors during the color filter array decoding process (which will soften detail slightly). Note however that too much in-camera sharpening will create sharpening halos and increase the visibility of jaggies, noise, and other image artifacts. Prosumer digital cameras and digital SLRs allow users to control the amount of sharpening applied to an image, or even disable it completely.
Sharpening with Software
If the camera allows you to shoot in RAW, the in-camera sharpening can be undone via software afterwards on your computer. You can then decide the level of sharpening you want to apply in order to avoid the above sharpening halos and depending on the purpose. For instance for web or monitor viewing purposes you may want to apply some sharpening to "pull out" fine details of downsampled images. For printing, sharpening should be applied with caution to avoid the image looking fake and over-processed.
If you shoot in JPEG it is recommended to apply some in-camera sharpening (e.g. "Low" or "Normal") because with regular software, it is not so easy to achieve the same sharpening quality level of in-camera sharpening. One of the reasons is that in-camera sharpening is applied before JPEG compression, while sharpening on your computer is done after JPEG compression, thereby making the edges of the JPEG compression squares more visible. If the in-camera sharpening was insufficient, you can still apply some additional sharpening with software. This is much easier than to undo the effects of over sharpening.
Image Resolution
The resolution of a digital image is defined as the number of pixels it contains. A 5 megapixel image is typically 2,560 pixels wide and 1,920 pixels high and has a resolution of 4,915,200 pixels, rounded off to 5 million pixels. It is recommended to shoot at a resolution which corresponds with the camera's effective pixel count. As explained in the pixels topic, shooting at higher (interpolated) resolutions (if available as an option) creates only marginal benefits but takes up more card space. Shooting at lower resolutions only makes sense if you are running out of card space and/or image quality is not important.
Resolution Charts at dpreview.com: Horizontal and Vertical LPH
We measure resolution using the widely accepted PIMA/ISO 12233 camera resolution test chart. This chart is excellent, not only for measuring pure horizontal and vertical resolution, but also to test the performance of the sensor with frequencies at various angles. It also offers a good reference point for comparison of resolution between cameras. The chart is available for every camera which comes through our test labs, both in the camera reviews and our extensive camera database.
Saturation How rich the colors are in a photo.
Sensitivity (ISO) Conventional film comes in different sensitivities (ASAs) for different purposes. The lower the sensitivity, the finer the grain, but more light is needed. This is excellent for outdoor photography, but for low-light conditions or action photography (where fast shutterspeeds are needed), more sensitive or "fast" film is used which is more "grainy".
Likewise, digital cameras have an ISO rating indicating their level of sensitivity to light. ISO 100 is the "normal" setting for most cameras, although some go as low as ISO 50. The sensitivities can be increased to 200, 400, 800, or even 3,200 on high-end digital SLRs. When increasing the sensitivity, the output of the sensor is amplified, so less light is needed. Unfortunately that also amplifies the undesired noise. Incidentally, this creates more grainy pictures, just like in conventional photography, but because of different reasons. It is similar to turning up the volume of a radio with poor reception. Doing so will not only amplify the (desired) music but also the (undesired) hiss and crackle or "noise". Improvements in sensor technology are steadily reducing the noise levels at higher ISOs, especially on higher-end cameras. And unlike conventional film cameras which require a change of film roll or the use of multiple bodies, digital cameras allow you to instantly and conveniently change the sensitivity depending on the circumstances.
Sharpening There are two types of sharpness and it is important not to mix them up. Optical sharpness is defined by the quality of the lens and the sensor. Software sharpness will create an "optical illusion" of sharpness by making the edges more contrasty. Software sharpening is of course unable to create detail beyond the camera's resolution, it will only help to bring out captured detail.
In-camera Sharpening
Digital cameras will, as a part of their default image processing, apply some level of sharpening, to counteract the effects of the interpolation of colors during the color filter array decoding process (which will soften detail slightly). Note however that too much in-camera sharpening will create sharpening halos and increase the visibility of jaggies, noise, and other image artifacts. Prosumer digital cameras and digital SLRs allow users to control the amount of sharpening applied to an image, or even disable it completely.
Sharpening with Software
If the camera allows you to shoot in RAW, the in-camera sharpening can be undone via software afterwards on your computer. You can then decide the level of sharpening you want to apply in order to avoid the above sharpening halos and depending on the purpose. For instance for web or monitor viewing purposes you may want to apply some sharpening to "pull out" fine details of downsampled images. For printing, sharpening should be applied with caution to avoid the image looking fake and over-processed.
If you shoot in JPEG it is recommended to apply some in-camera sharpening (e.g. "Low" or "Normal") because with regular software, it is not so easy to achieve the same sharpening quality level of in-camera sharpening. One of the reasons is that in-camera sharpening is applied before JPEG compression, while sharpening on your computer is done after JPEG compression, thereby making the edges of the JPEG compression squares more visible. If the in-camera sharpening was insufficient, you can still apply some additional sharpening with software. This is much easier than to undo the effects of over sharpening.
T
Thumbnail A small version of a photo. Image browsers commonly display thumbnails of photos several or even dozens at a time. In Windows XP's My Pictures, you can view thumbnails of photos in both the Thumbnails and Filmstrip view modes.
TIFF TIFF (Tagged Image File Format) is a universal image format that is compatible with most image editing and viewing programs. It can be compressed in a lossless way, internally with LZW or Zip compression, or externally with programs like WinZip. While JPEG only supports 8 bits/channel single layer RGB images, TIFF also supports 16 bits/channel multi-layer CMYK images in PC and Macintosh format. TIFF is therefore widely used as a final format in the printing and publishing industry.
Many digital cameras offer TIFF output as an uncompressed alternative to compressed JPEG. Due to space and processing constraints only the 8 bits/channel version is used in digital cameras. Higher-end scanners offer a 16 bits/channel TIFF option. If available, RAW is a much better alternative for digital cameras than TIFF.
Tonal Range The tonal range of a digital image is the number of tones it has to describe the dynamic range. These conceptual examples show that an image with a large dynamic range can have a narrow tonal range and an image with a low dynamic image can have a wide tonal range.
Dynamic Range and Tonal Range of the Sensor The dynamic range and tonal range of a sensor are related. If a sensor has a dynamic range of say 1000:1 AND it has an ADC of at least 10 bit, it automatically has a wide tonal range. If a sensor with a 10 bit ADC is able to output about 1,000 different tones, the sensor must have a dynamic range of at least 1000:1. This is because the sensor is linear and an ADC samples in equal steps.
Dynamic Range and Tonal Range of the Image Once you apply a tonal curve to the linear sensor data, the dynamic range and tonal range of the image can vary independently, depending on what tonal curve you apply. The tonal curve can compress the dynamic range, the tonal range, or both (*).
When shooting in JPEG, the rather contrasty tonal curves applied by the camera may clip shadow and highlight detail which was present in the RAW data. RAW images preserve the dynamic range captured by the sensor and allow you to compress the dynamic range and tonal range by applying a proper tonal curve so that the whole dynamic range is represented on a monitor or print in a way that is pleasing to the eye. This is similar to the more extreme example below which shows how the larger dynamic range and tonal range of a 32 bit floating point HDR image were compressed.
Dynamic Range and Tonal Range of a Monitor or Printer - Compression Monitors and printers have a limited dynamic range. Therefore a tonal curve is applied to the linear raw data to compress the dynamic range so that it fits in the dynamic range of the monitor or printer. The tonal curve is chosen so that detail is preserved where it is most noticeable. As a result, the image looks pleasing to the human eye and the perceived dynamic range is higher, even on a limited dynamic range print.
TIFF TIFF (Tagged Image File Format) is a universal image format that is compatible with most image editing and viewing programs. It can be compressed in a lossless way, internally with LZW or Zip compression, or externally with programs like WinZip. While JPEG only supports 8 bits/channel single layer RGB images, TIFF also supports 16 bits/channel multi-layer CMYK images in PC and Macintosh format. TIFF is therefore widely used as a final format in the printing and publishing industry.
Many digital cameras offer TIFF output as an uncompressed alternative to compressed JPEG. Due to space and processing constraints only the 8 bits/channel version is used in digital cameras. Higher-end scanners offer a 16 bits/channel TIFF option. If available, RAW is a much better alternative for digital cameras than TIFF.
Tonal Range The tonal range of a digital image is the number of tones it has to describe the dynamic range. These conceptual examples show that an image with a large dynamic range can have a narrow tonal range and an image with a low dynamic image can have a wide tonal range.
Dynamic Range and Tonal Range of the Sensor The dynamic range and tonal range of a sensor are related. If a sensor has a dynamic range of say 1000:1 AND it has an ADC of at least 10 bit, it automatically has a wide tonal range. If a sensor with a 10 bit ADC is able to output about 1,000 different tones, the sensor must have a dynamic range of at least 1000:1. This is because the sensor is linear and an ADC samples in equal steps.
Dynamic Range and Tonal Range of the Image Once you apply a tonal curve to the linear sensor data, the dynamic range and tonal range of the image can vary independently, depending on what tonal curve you apply. The tonal curve can compress the dynamic range, the tonal range, or both (*).
When shooting in JPEG, the rather contrasty tonal curves applied by the camera may clip shadow and highlight detail which was present in the RAW data. RAW images preserve the dynamic range captured by the sensor and allow you to compress the dynamic range and tonal range by applying a proper tonal curve so that the whole dynamic range is represented on a monitor or print in a way that is pleasing to the eye. This is similar to the more extreme example below which shows how the larger dynamic range and tonal range of a 32 bit floating point HDR image were compressed.
Dynamic Range and Tonal Range of a Monitor or Printer - Compression Monitors and printers have a limited dynamic range. Therefore a tonal curve is applied to the linear raw data to compress the dynamic range so that it fits in the dynamic range of the monitor or printer. The tonal curve is chosen so that detail is preserved where it is most noticeable. As a result, the image looks pleasing to the human eye and the perceived dynamic range is higher, even on a limited dynamic range print.
U
USB Universal Serial Bus: a protocol for transferring data to and from digital devices. Many digital cameras and memory card readers connect to the USB port on a computer. USB card readers are typically faster than cameras or readers that connect to the serial port, but slower than those that connect via FireWire.
W
White Balance and Color Temperature
Most light sources are not 100% pure white but have a certain "color temperature", expressed in Kelvin. For instance, the midday sunlight will be much closer to white than the more yellow early morning or late afternoon sunlight. This diagram gives rough averages of some typical light sources.
Type of Light Color Temperature in K
Candle Flame 1,500
Incandescent 3,000
Sunrise, Sunset 3,500
Midday Sun, Flash 5,500
Bright Sun, Clear Sky 6,000
Cloudy Sky, Shade 7,000
Blue Sky 9,000
White Balance
Normally our eyes compensate for lighting conditions with different color temperatures. A digital camera needs to find a reference point which represents white. It will then calculate all the other colors based on this white point. For instance, if a halogen light illuminates a white wall, the wall will have a yellow cast, while in fact it should be white. So if the camera knows the wall is supposed to be white, it will then compensate all the other colors in the scene accordingly.
Most light sources are not 100% pure white but have a certain "color temperature", expressed in Kelvin. For instance, the midday sunlight will be much closer to white than the more yellow early morning or late afternoon sunlight. This diagram gives rough averages of some typical light sources.
Type of Light Color Temperature in K
Candle Flame 1,500
Incandescent 3,000
Sunrise, Sunset 3,500
Midday Sun, Flash 5,500
Bright Sun, Clear Sky 6,000
Cloudy Sky, Shade 7,000
Blue Sky 9,000
White Balance
Normally our eyes compensate for lighting conditions with different color temperatures. A digital camera needs to find a reference point which represents white. It will then calculate all the other colors based on this white point. For instance, if a halogen light illuminates a white wall, the wall will have a yellow cast, while in fact it should be white. So if the camera knows the wall is supposed to be white, it will then compensate all the other colors in the scene accordingly.