The main goal of digital color imaging is to CAPTURE as much color information as possible, and to PRESERVE that color information throughout the editing processes.

That said, nothing is more damaging, destructive to colour information than editing it.

Whether we do the editing, color correcting, image adjustments in Photoshop's Curves, Selective Color, Levels, Channel Mixer, Hue/Saturation, or Color Balance — we need to develop strategies of high–bit CAPTURE and least–destructive color PRESERVATION.

The high–bit, 16–bit scanning Workflow gballard.net, and Adobe's Camera RAW Photoshop plug-in gballard.net provide high–bit capturing methods, but this how to tutorial is about the least–destructive editing methods, specifically, Photoshop's Adjustment Layers.

The first "rule" to learn is:

You move (in Photoshop), you lose (information you cannot get back).

We can witness this fact first hand in Photoshop's Levels, Histogram:

Original Capture: Notice the full (healthy) continuous black area — this is what we want.
A Minor Gamma Correction center slider moved (to slightly lighten the image).
Edit Applied: Notice the gaps of missing information and degraded height, the file is actually missing color information in those empty levels — this is what we want to avoid.

In a photo, those holes, blanks, gaps, missing areas represent posterization in the image.

Make a couple more minor Edits — outside of Adjustment Layers — and it is easy to see how quickly even slight moves add up in Photoshop to significant image degradation.

+++++++

ENTER ADJUSTMENT LAYERS:

		like to visualize adding an Adjustment Layer in Photoshop is like adding a photographic filter over a camera lens (or adding a color-correcting gel over a photograph to evaluate the adjustment). In other words, adding an 81a warming filter onto a camera lens is similar to adding the same correction in an Adjustment Layer above an image layer in Photoshop. We can stack our Adjustment Layers — archive the file with the Adjustment Layers in place — PLUS, in a nonlinear editing fashion — we can go back and reedit the individual Adjustment Layers at any time (never touching our original image layer(s). We can apply, flatten, merge all the Adjustment Layers in one fell swoop, which — as the Histogram will prove — is by far PROFOUNDLY the least destructive editing WorkFlow available.
Iused to like to think of the flattening, merging process as Photoshop crunching a gigantic algebra equation (but have since found out Photoshop CS2 CS1 9 8 7 6, in fact, computes each adjustment individually so the only benefit is that we can save the edits and re edit the later). Still, it was an interesting thought ;) All the various Adjustment Layers will contain common algebraic variables (numbers) that are solved by opposite operations (they cancel each other out), and the end result is the smallest possible shift in the numbers, PRESERVING the greatest amount of original information....