Levels vs Curves - The Midtone Slider

What about the midtones? This is where Photoshop's Curves adjustment starts to get interesting. In Levels, we have a midtone slider that we can drag left or right to lighten or darken the middle brightness values in the image, but at first, Curves doesn't seem to have anything like that. We've already seen that the points on either end of the diagonal line are the Curves equivalent of the black point and white point sliders in Levels, but there are no other points on the line anywhere to be found. At least, not yet there isn't!

What makes Curves so incredibly powerful and useful, far more than the Levels command, is that it allows us to add points wherever we want! For example, let's say we want a point that will give us the same control over the midtones that we get with the midtone slider in Levels. All we need to do is add it! To add a point, simply click on the line at the spot where you want the point to be added. To gain the same control over the middle brightness values that I'd get with the midtone slider in Levels, I'll click to add a point directly in the center of the line. Notice that as soon as I add the point, both the Input and Output readings show a value of 128, which tells me that I've just added a point at brightness level 128 and since both readings are showing the same value, I haven't yet made any changes:

So far, we've been dragging the points either left or right. This changed the brightness level we selected in the image (the Input level) but left the Output level unchanged. Dragging the black point to the right allowed us to select every brightness value up to level 20 (the Input level) and force it down to level 0 (the Output level), while dragging the white point to the left allowed us to select every brightness value down to 232 (Input level) and force it up to 255 (Output level). For our midtone point, we want the exact opposite. We want to leave our Input value at 128 (or something close to 128 if you didn't click exactly in the middle) and change the Output level. We do that by dragging the point up or down! Drag the point up to increase the Output value, which will lighten the brightness level you selected, or drag it down to darken the brightness level.

I want to darken my midtones, just as I did with the Levels command, so I'm going to drag my new point straight down. Like the midtone slider in Levels, you don't need to drag points very far to get noticeable results so you'll definitely want to keep an eye on your image as you drag points around. Unlike the midtone slider in Levels which deals with gamma values, Curves keeps things simple by dealing only with brightness levels, so you can safely forget all about gamma values when working in Curves. I'm going to drag my middle point from an original value of 128 down to around 110:

This takes any areas of the image that were originally at a brightness level of 128 and darkens them to a new level of 110. Notice that by dragging the middle point, our diagonal line is now bending into a curve (which finally explains why this is called the Curves command). The curve tells us that every other brightness level along the line is also being adjusted as we drag points around to reshape the curve. So even though we haven't actually selected any other brightness levels, they've all been affected to some degree by the adjustment.

Here's my image after darkening the midtones with Curves. The effect is exactly the same as if I had darkened them with the midtone slider in Levels:

Going Beyond Levels With Curves

At this point, we've seen how we can accomplish the exact same basic tonal adjustments with Curves that we can with Levels. We can drag the black point in Curves to darken the shadows, drag the white point to brighten the highlights, and add our own point in the middle of the line to lighten or darken the middle brightness values. You may be wondering what the big deal is with Curves if we can make the same adjustments with Levels, especially when Levels seems so much easier to understand. Well, the big deal is that we can go even further with Curves! A lot further! Levels gives us only three adjustments - a black point, a white point and a single midtones slider. Curves, on the other hand, gives us as much control as we want. We can add up to 16 individual points along the curve for pinpoint accuracy when adjusting brightness values!

For example, one of the most common uses for Curves is to improve contrast in an image by adding what's called an "S" curve, meaning that we reshape the curve into something that resembles a letter S. For that, we'll need to add a couple more points, one around the 3/4 tone mark to darken the shadows even further and one around the 1/4 tone mark to brightness the highlights more. There's no way we could do this with Levels, but with Curves, it's easy. First, I'll click on the line roughly half way between the black point and the middle point I added earlier. This adds a fourth point. Then I'll drag the new point downward a short distance to darken the shadow areas even more:

I'll do the same thing with the highlights. I'll click on the line roughly half way between the middle point and the white point, which adds a fifth point to the curve. Then, to brighten the highlights even further, I'll drag the new point a short distance upward:

If you need to re-adjust any of the points you've already added, simply click on the point to select it, then drag it up, down, left, right, or even diagonally as needed. Dragging a point diagonally will affect the Input and Output levels at the same time. In my case, I think my middle brightness values are now a little too dark, so I'm going to click on my middle point to re-select it and drag it back up to its original brightness value of 128:

With the curve now looking similar to a letter S (sort of), we've managed to increase contrast in the image even further:

Depending on the image you're working on, you may be able to lighten the highlights and darken the shadows even more than I've done here, creating a more pronounced S curve. In my case though, increasing the contrast further would have made the image look too harsh, so I made only minor adjustments.

By default, the Levels and Curves commands affect both the luminosity (brightness) and color saturation of an image. In most cases, this is what you want, but if you find that your colors are becoming too saturated and you want to limit the effect of the adjustments to just the brightness values, simply go up to the top of the Layers palette and change the blend mode for the adjustment layer from Normal to Luminosity:

With the blend mode set to Luminosity, the colors have returned to their original saturation values while the increased contrast remains:

And there we have it! That's our first look at Photoshop's Curves command! We've seen how to make basic tonal adjustments to an image with Levels, how to make the same adjustments with Curves, and how we can then go even further with Curves by adding additional points to the shadows and highlights. We've also seen how to limit the effects of the Levels and Curves commands to just the brightness levels by changing the blend mode to Luminosity. We'll continue to explore the full power of Curves in more tutorials, including a look at the added features available in Photoshop CS3 and CS4.