What's a RAW file?

Some insight about how digicams work is needed to understand this:

Digital Cameras have sensors (all that CCD, CMOS and Foveon babble you see on forums and technical articles) to capture the light that comes from their lens.

These sensors (but the Foveon) cannot see any colors, but different shades of grey. However, they record those seperately for each additive primary color, i.e. Red, Green and Blue.

When you press the camera shutter release, and your camera is adjusted to shoot in a graphic format (i.e. JPEG, TIFF, etc) the sensor captures those three layers of grey values for each color, and then passes all that information to the camera processor which in turn, using image processing algorithms:

  1. Reconstructs the color information from the RGB layers grey values
  2. Reconstructs the brightness (sensors doesn’t see like humans, but on a very dark manner).
  3. Adjust colors according to the White Balance set on the camera
  4. Applies sharpening (due to the RGB filtering on the 1st stage, everything is blurred) so that things appear crisp if they should;
  5. If you’re shooting
  • JPEG: the resulting image is compressed (i.e. most part of that data is discarded) using JPEG’s lossy algorithm, EXIF information is embedded and the resulting file is then saved to your camera’s memory media.

Quick file size rule of thumb: camera’s megapixels divided by three (3MP digicam~1Mb file).

  • TIFF: the resulting image is tagged with the EXIF data, and then directly saved to your camera’s memory media, occupying an awful amount of space (quick rule of thumb: megapixels multiplied by 4; i.e. 3MP camera ~ 12Mb TIFF ).

So, what happens when you shoot in RAW format?

Data from the sensor is not processed in any way; instead, is compressed with a lossless algorithm (like those that winzip, stuffit et al employ) and then saved to the memory media.

(EXIF information is either embedded on a seperate file or into the RAW file)

RAW file size rule of thumb: about the same megabytes as Megapixels the sensor has: 6 MP camera = 6Mb RAW file.

That’s all: no processing at all. All the processing you see happening on the JPEG shooting is done later, on your computer, using specific software tools that will mimic what the camera will have done, but following your directions; if you mistakenly shoot at the beach with the tungsten white balance setting (everything is blue blue blue!), just change it to sunny and voilĂ : colors obey and turn into their proper tones.

Doing all these processing offline means also that you can do it with different, and possibly more sophisticated software, and more processing power. And if done right, extract the max out of your camera.

RAW files are Undeveloped Digital Film that doesn’t age. I won’t be surprised if improved software appears in the future correcting problems with specific cameras, or simply enhancing their RAW files processing, obtaining better image quality. This has in fact already happened to Canon EOS D60 owners, making reprocessing all their RAW files worthy (enhanced WB).


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