This is an extract from “Understanding Digital Raw Capture” which was written by Bruce Fraser. It was adapted from his book Real World Camera Raw, published by Peachpit Press, in August, 2004. It is quite interesting to see how the Raw files work using metadata to transform the greyscale captured image into a full colour composite, not throwing any information away. And how you can loose information by using JPEG and its tendency to have a steep contrast curve to make images look like film colour slides.

Raw files contain two different types of information: the image pixels themselves, and the image metadata. Metadata, which literally means “data about data,” is generated in the camera for each capture. Both raw and JPEG captures, for example, contain EXIF (Exchangeable Image Format) metadata that records shooting data such as the camera model and serial number, the shutter speed and aperture, the focal length, and whether or not the flash fired. Raw files also include some additional metadata that raw converters need in order to process the raw capture into an RGB image.

In addition to the grayscale values for each pixel, most raw formats include a “decoder ring” in metadata that conveys the arrangement of the color filters on the sensor, so it tells raw converters which color each pixel represents.The raw converter then uses this metadata to convert the gray-scale raw capture into a color image by interpolating the “missing” color information for each pixel from its neighbors.

How JPEG differs from raw

When you shoot JPEG, a raw converter built into the camera carries out all the tasks listed earlier to turn the raw capture into a color image, then compresses it using JPEG compression. Some cameras let you set parameters for this conversion—typically, a choice of sRGB or Adobe RGB as color space, a sharpness value, and perhaps a tone curve or contrast setting. Unless your shoot- ing schedule is atypically leisurely, it’s difficult to adjust these parameters on an image-by-image basis, so you’re locked into the camera’s interpretation of the scene.

When you shoot raw, however, you get unparalleled control over the interpretation of the image through all the aforementioned aspects of the conversion. When you shoot raw, the only on-cam- era settings that have an effect on the captured pixels are the ISO speed, the shutter speed, and the aperture setting. Everything else is under your control when you convert the raw file—you can reinterpret the white balance, the colorimetric rendering, the tonal response, and the detail rendition (sharpening and noise reduction) with a great deal of freedom. Within limits (which vary from one raw converter to another), you can even reinterpret the exposure compensation.

Almost all cameras that shoot raw capture at least 12 bits, or 4096 shades, of tonal information per pixel. The JPEG format, however, is limited to 8 bits per channel per pixel, so when you shoot JPEG, you’re trusting the camera’s built-in raw converter to throw away a large amount of the captured data in a way that will hopefully do the image justice. This is exacerbated by the tendency of most camera vendors to impose a fairly steep contrast curve in the raw-to-JPEG conversion in an effort to produce a JPEG that resembles a transparency. In the process, they throw away about a stop of usable dynamic range, and you have essentially no control over what gets discarded.

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