The Canon EOS Digital Rebel XSi/450D Companion

CHAPTER 11

WHAT IS RAW?

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What Is Raw?

In Chapter 7, you saw how, with JPEGs, the camera takes the data off the image

sensor and performs a number of processes to it, from white balance adjustment

to gamma correction to a number of corrections and adjustments similar to what

you might apply in an image editor. You also learned that one of the last things

the camera does before saving a JPEG image is to throw out a lot of color data so

that the 14-bit image that the camera captured can be stored in the 8-bit space

that the JPEG format specification dictates. Earlier, I also told you about JPEG

compression and how it is a lossy compression scheme that can, when used ag-

gressively, degrade your image.

While the XSi’s high-quality JPEG compression is very good—you’ll be hard-

pressed to find evidence of JPEG compression in an XSi file saved at high quality

—if you later perform a few edits in your image editor and save the file again as

a JPEG, then you might start seeing some blocky patterns. Similarly, though the

camera has to toss out 6 bits of data per pixel when it saves to JPEG, you won’t

necessarily notice a lack of color in the JPEG files saved by the camera. However,

once you start editing and begin to brighten or darken areas, you might notice

that gradient areas of your image (skies or reflections on a curved surface, for ex-

ample), start to look chunky as you push your ...