Some Random Thoughts on Superman and Photoshop Curves
Superman is famous for the giant "S" emblazoned on the chest of his signature outfit. I doubt it has anything to do with the ability of an S-curve in Photoshop to make images look better, but both are legendary for the way they help out those in need. OK, so Photoshop may not be faster than a speeding bullet or more powerful than a locomotive, but the S-curve is a remarkable tool that you really should be familiar with.
For the uninitiated, Curves allow you to remap tones from dark to light. The Curves interface is deceptively simple. The bottom axis represents the range of brightness of your unmodified image with pure black on the left and end ranging to pure white on the far right. In a similar way, the vertical axis represents a scale of black to white for your transformed image. The line spanning the diagonal of the graph formed by these two axes is the curve itself and initially maps every input value across the bottom to the exact same value up the side. Black maps to black, white maps to white, and everything in between maps to itself as well. When you move the line though, you change that mapping and thereby change how at least some input values end up in the edited image.
Before applying S-curve
After applying S-curve
But in that simplicity lies a frustrating complexity too. It is quite common for new Photoshop users to open the Curves dialog to see what it is capable of, only to end up clicking on "Cancel" after they horribly mess up the image they are working on. Using Curves is easy. Using it well is not — at least not at first.
Superman's powers supposedly come from exposure to Earth's yellow sun together the weaker gravity here compared to that on Krypton. This never made much sense to me, but that's more or less the official story. For most Photoshop users that use Curves, the power of a simple S-curve can be equally hard to explain.
The key to Curves is that slope equals contrast. The initial curves line has a constant slope going uphill from lower left to upper right. Move a portion of that line to make the slope steeper and you will have increased the contrast over the portion of the image within that brightness range. But since the line has to stay within the area of the graph, increasing the slope in one place inherently means lessening the slope somewhere else. There's no two ways about it. The line can't travel any farther in its journey from bottom to top than the distance across the curves grid so if it covers part of that distance more quickly it has to slow down elsewhere.
The more points on curve line you grab and move, the more detailed the resulting curve will become and the more control over how contrast is remapped you will be exercising. But even a modest change to the curve line can have a huge impact on the appearance of an image. The general example of such a powerful but modest curve can be formed by slightly pulling up on a point to the right of center and slightly down on one to the left. The rest of the line will bend to avoid sharp corners resulting in a curve shaped something like the letter "S." The slope between the two points you move to create an S-curve will be increased resulting in an increase of contrast in the mid-tones of the image. Contrast in the shadows and highlights that lie outside the two points will be correspondingly decreased — an often acceptable tradeoff since detail in these regions is generally less anyway. With the exposure of your main subject typically falling somewhere between the two points creating the S-curve, increasing mid-tone contrast will give your subject more "pop" with subject details standing out better due to the increased contrast.
But a little goes a long way with Curves, so don't overdo it. Superman must know his own strength or he'll risk crushing someone when trying to save them. The power of Curves can easily be overdone too if you aren't careful. Pull the curves line too far and you can ruin an image by accentuating not only the detail but also the noise and other digital artifacts to create an unnatural result.
Negative slope in Curves is like green kryptonite. When the slope goes downhill rather than up, Curves maps brighter tones to darker ones and vice versa. This creates a sort of inverted "negative" look, sapping the strength from your image and turning it into something quite unnatural. Avoid downward slope as you would kryptonite. Only criminal geniuses like Lex Luthor who deal in weird Photoshop special effects create curves with negative slope, not us photographers.