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A camera raw image file contains the unprocessed data from the image sensor of a digital camera. Also called RAW (although it is not an acronym) or CCD-RAW (even for CMOS sensors), its format is proprietary and differs from one manufacturer to another, and sometimes between cameras made by one manufacturer. The image must be processed and converted to an RGB format such as TIFF or JPEG before it can be manipulated by a bitmap graphics editor, printed, or displayed on a web site.
Nearly all digital cameras can process the image from the sensor into a JPEG file using settings for white balance, color saturation, contrast, and sharpness that are either selected automatically or entered by the photographer before taking the picture. Cameras that support raw files save these settings in the file, but defer the processing. This results in an extra step for the photographer, so raw is normally only used when additional computer processing is intended. However raw permits much greater control than JPEG for several reasons:
- Finer control is available for the settings when a mouse and keyboard are available to set them. For example, the white point can be set to any value, not just discrete values like "daylight" or "incandescent".
- The settings can be previewed and tweaked to obtain the best quality image or desired effect. (With in-camera processing, the values must be set before the exposure.) This is especially pertinent to the white balance setting since color casts can be difficult to correct after the conversion to RGB is done.
- Most camera raw files have a color depth of 12 bits per pixel instead of the 8 used by JPEG. This allows minor exposure errors to be corrected and tonal changes to be made with less risk of posterization. (However, overexposed areas are just as white with 12 bits as with 8, so using raw is not a substitute for correct exposure.)
- Different RGB conversion algorithms can be used, not just the one coded into the camera.
Camera raw files are 2-4 times larger than JPEG files since lossless data compression is used. This avoids the compression artifacts inherent in JPEG, but means that less images can fit on a given memory card. It also takes longer for the camera to write raw images to the card, so fewer pictures can be taken in quick succession (affecting the ability to take, for example, a sports sequence).
Cameras that support raw files typically come with proprietary software for conversion or their raw format to TIFF or JPEG. Other conversion programs and plugins are available from vendors that have either licensed the technology from the camera manufacturer or reverse-engineered the particular raw format. A portable open source program, dcraw, supports most raw formats and can be made to run on operating systems such as Unix not supported by most commercial software.
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