graphics tips » Photoshop's S-Curves

What is the best way to adjust a photo? S-Curves!

"Best" is impossible as every image differs considerably, but some ways are better than others. There are three primary ways within Photoshop to adjust photos, so let's look at them starting with the simplest.

First, Brightness & Contrast. This is a very crude, down and dirty adjustment that is, in fact, a dumbed down version of Levels, so quickly wean yourself off this one because it rarely produces happy results. Next: Levels.

Levels is very good, definitely better than Contrast, and for evaluating a scene (that is, informational purposes) it is perhaps the best measure of your success in capturing the shot (I shoot with my camera's display set to always show the levels histogram (thanks, Paul) -- the best way confirm that you're not clipping off detail). But, in truth, Levels suffers from a couple of significant drawbacks. First, the gamma on Macs and Windows boxes differ by a few tenths (may not sound like much, but a tenth is a lot). Thus levels adjusted on a Mac often are too dark when viewed on Windows, and those adjusted on Windows are almost always too light (flat) on a Mac. Also, Levels gives you three points, which often, no matter how much tweaking you employ, just doesn't cut it. So what else is there?

Photoshop S-CurveThe answer: Curves. Curves is to Levels as Levels is to Brightness & Contrast: Curves is a souped-up Levels control. Plus, Curves can accommodate some fairly drastic color-balance issues by adjusting each channel's Curve independently (for example, photos shot on overcast days or in shadows can easily be fixed by slightly pulling down the blue curve). But the most useful "trick" you can perform with Curves is a simple "S"-shaped curve. This will pop the contrast and the saturation, giving very pleasing rich blacks and crisp whites.

In this snapshot of the Curves dialog box the blue line is the default -- output levels are exactly mapped to input levels. The black (rough) line is a moderate S-curve, the red line a rather pronounced curve. The two adjusted curves bump-up brights, and pull-down the darks (output level greater than input, output level less than input respectively). Note that in the middle areas the curve is nearly linear (flat 1:1) so midtones are barely affected.

Here's an example where an acceptable photo undergoes dramatic changes (some of you won't like):
S-Curve Bird of Paradise Demo

Resources for another explanation:
Digital Outback Photo: The Power of the S-Curves