1. THIS IS MOST IMPORTANT: Data is (are?) not the same as information. Data are bit values. Information is what the photograph tells you visually. You may lose data and not lose information. In fact you may gain information. Untouched data is often not higher-quality information. What you care about is information. A trivial example: when you white balance an image file, the color-corrected photo that you save has sacrificed some of your original data, because some RGB value ranges were compressed with others were expanded. But it gains you useful information on the colors of objects in the photograph and you don't lose any other information.
2. Less editing is better than more because editing can lose information. That's not the same as will. We teach students to do as little editing as possible to minimize the chances they'll degrade their information, and because really excessive fiddling will cause visible information loss. But it's just a good workflow principle, especially for newbies, not a rule of nature (or even art). Plus...
3. Tonal editing done in 16-bit mode almost never loses data or information, no matter how extreme. (Spatial editing is another matter.) I recommend people with enough computer muscle always work in 16-bit mode, even if their starting point is an 8-bit file. Makes for less worrying about such losses.
4. Layers do not change the nature of the data and information manipulations you've done. They preserve the original file unchanged, but in terms of the output result (on screen or printer), the same adjustment applied directly to a file and applied in a layer has precisely the same effect on the photo. (By the way, you do not need to flatten a layered file [or convert from 16-bit to 8] before printing. Photoshop flattens and converts the data as it renders it for printing.)
5. Doing all your editing in a single session in Photoshop does not produce better data or information. So long as you save as TIFF or PSD, it doesn't matter whether you do your editing in one session or twenty before printing.
6. Low-compression ratio JPEG's won't cost you any visible spatial detail, but JPEG doesn't support 16-bit color. Avoid saving your files as JPEG unless you don't care about losing your 16-bit data.
Posted by: CTEIN
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