I would like to share some wisdom that I recently gained on the Visual Resource Association List serve. Ever wonder what a RAW image really is? Alex Nichols, Michigan State University provided one of the most coherent explanations that I have seen and which he has agreed to let me share with you here. In Response to the question:" I don’t understand why RAW originals are to be left as is and all editing done on the copy?" , he wrote:

You can't really edit and save a raw file -- that would defy its "raw" nature.

The simple explanation: Imagine a tiff file as a house, and a raw file as a pile of lumber and aluminum siding with blueprints laying on top. After you follow the blueprints to make the pile of lumber and siding into a house, you can't still save it as a pile of lumber and siding.

The elaborate explanation: A raw is not like a normal image file format. A normal image file (like a tiff) is made up of a grid of pixels, each pixel having a color determined by three (assuming it is a RGB color image) separate grayscale values. Each of the grayscale values represents the red, blue, or green channel, and when combined, give the pixel its color. So a 6 million pixel image actually consists of 6x3=18 million pieces of color information.

A raw file (we'll assume from a conventional camera sensor) is still made up of a grid of pixels, but each of the six million pixels only has one piece of color information: 3 million of the pixels have grayscale information representing the green channel, 1.5 million represent the red channel, and 1.5 million represent the blue channel. The pixels are arranged GRGBGRGBGRGB..., based on the pattern of color filters on the sensor. Included in the raw file, are intructions from the camera about how the color information might be interpreted (white balance, saturation, sharpness, etc.).

When you open the raw file in Photoshop, the Adobe Camera Raw plugin is launched, and will show you various settings (including some that are the instructions from the camera, and others that are suggested by ACR) which will determined how the pixel color information will be interpreted in the final image (plus there is a preview image).

When you finish your adjustments in ACR and hit the ok button, the raw file is "demosaiced" and opened in Photoshop. "Demosaicing" involves using the settings you've selected, plus color information from neighboring pixels to give each pixel three separate channel values (red, blue, and green). Once this is done, you can work on the image normally in Photoshop, but when you're ready to save, you must save it as a tiff or other normal file format, because to save it as a true raw file, it would need to be un-demosiaced into single channel pixels again, and you would lose all the work you did on the image (no program will do this).

There are a couple of new raw processors out from Adobe and Apple that appear to let you save all your changes to the raw file, but they are actually saving a list of instructions for demosaicing the the raw file and leaving the actual file untouched.

--Alex

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Alex Nichols
Academic Technology Coordinator
Department of Art and Art History
Michigan State University