Written By Steve Patterson

In this photo editing tutorial, we'll look at the similarities between two of Photoshop's most important image editing tools - the Levels and Curves adjustments. At first glance, the Curves adjustment may seem less like an image editing tool and more like something only a scientist would know what to do with, especially when compared with the much simpler Levels adjustment which we looked at earlier. Don't let it fool you, though. Curves may be extremely powerful, going far beyond what can be accomplished with Levels, but once you understand how it works, Curves is actually very simple. In fact, it's as simple as, well, drawing curves! In this first look at Curves, we'll compare it with the Levels command to see just how similar the two really are. This will give us a good starting point for a more in-depth look at Curves later, including how the Curves adjustment has changed and improved in more recent versions of Photoshop (CS3 and CS4). You may want to read through the Improving Image Tone With Levels tutorial before continuing on if you're completely unfamiliar with Levels. It may also help to read through the How To Read A Histogram tutorial if you're unsure of how histograms work in Photoshop.

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As I mentioned, we'll save the discussion on the newer features of the Curves dialog box in Photoshop CS3 and CS4 for later. We're just going to look here at the basics of Curves and how it compares with the Levels command. For that, I'll use Photoshop CS2, but any recent version will work.

Here's the image I have open on my screen:

A quick look at Photoshop's Histogram palette, which by default is grouped in with the Navigator and Info palettes (you can also select the Histogram palette from the Window menu if it's not appearing on your screen), shows us that there's definitely some room for improvement with the image. The main part of the histogram does not extend to either the far left (pure black) or far right (pure white), which tells us that we currently have no real shadows or highlights in the photo. Everything is bunched up in the midtones, resulting in a lack of overall image contrast:

Let's quickly improve the tonal range of the image using Photoshop's Levels adjustment. When we're done, we'll see how to make the exact same changes and more with Curves. First, I'll bring up a Levels adjustment layer by clicking on the New Adjustment Layer icon at the bottom of the Layers palette and selecting Levels from the list of adjustment layers that appears. As always, we're using adjustment layers rather than Photoshop's standard image adjustments because adjustment layers allow us to work flexibly and non-destructively on our images:

This brings up the Levels dialog box. As we looked at previously in the Levels tutorial, the Levels adjustment allows us to make three basic changes to improve overall image tone. We can brighten the highlights by setting a new white point, we can darken the shadows by setting a new black point, and we can lighten or darken the midtones in the image. The Levels dialog box makes these changes easy because it contains the exact same histogram that we saw a moment ago in the Histogram palette, and all we need to do to make adjustments to the image is drag the white point, black point and midtone sliders directly below the histogram:

The Levels dialog box also contains a handy black-to-white gradient below the histogram, making it easy to see exactly where the tonal range of the image falls. The higher the spike in the histogram directly above a certain brightness level in the gradient, the more pixels we have in the image at that brightness level compared with the number of pixels at other brightness levels. If no part of the histogram appears over a brightness level in the gradient, we know that we currently have no pixels in the image at that brightness level.

If you look directly above the histogram, you'll see three input boxes. These boxes show us the current black, white and midtone levels. The box on the left, with a default value of 0, shows us the current black point level. The box on the right, with a default value of 255, shows us the current white point level:

Photoshop divides the maximum tonal range of an image into 256 different brightness levels, with pure black being 0 and gradually increasing in brightness until you reach pure white at 255. By dragging the black and white point sliders below the histogram, we can change which brightness values become pure black or white. This stretches the current tonal range of the image, basically pulling the darkest areas to pure black and pushing the lightest areas to pure white, improving the photo's overall appearance. I'm going to set a new black point by clicking on the black point slider and dragging it in to the left edge of the histogram. Then I'll set a new white point by clicking on the white point slider and dragging it in to the right edge of the histogram. If we look again at the input boxes above the histogram, we can see that the values for the black point and white point have now changed, with the black point having increased from 0 to 20 and the white point now lowered from 255 down to 232:

What this means is that any areas in the image that were originally at a brightness level of 20 (a dark gray) have been pulled down to pure black. Any areas that were originally at a brightness level of 232 (a light gray) have been pushed up to pure white. The rest of the brightness values in the image have also shifted accordingly as the entire tonal range of the image is stretched to the full 0-255 range.

If we look in the Histogram palette, we can see what's happened. The histogram now extends all the way from the far left to the far right, which means we now have a full range of tonal values from pure black to pure white. The only downside is that by stretching the tonal information, we've lost some image detail, represented by a comb-like pattern of white vertical bars in the histogram. Each white vertical bar means that we no longer have any pixels in the image at that brightness level, but that's okay since we haven't lost enough detail for there to be any visible problems in the image itself:

In fact, not only are there no visible problems in the image, we've managed to improve its overall tone and contrast quite a bit simply by setting new black and white points in the Levels dialog box. The colors have also become more saturated:

The Levels command also allows us to lighten or darken the middle brightness levels in the image by dragging the midtone slider. Dragging the midtone slider towards the left will lighten the midtones, while dragging the slider towards the right will darken them. Technically, the middle slider is known as the gamma slider, and if you look in the middle input box above the histogram, you'll see that by default, it shows a value of 1.00, whereas you may have expected it to show a value of 127 or 128 (directly between black at 0 and white at 255). That's because it's telling us the current gamma value rather than the middle brightness value. Don't let the terminology confuse you, though. Gamma is just a strange sounding word meaning the overall brightness of an image. All you need to know is that no matter what you prefer to call it, the middle slider in the Levels dialog box lightens or darkens the midtones. I'm going to darken my midtones just a little to add a bit more contrast to the image by dragging the slider towards the right, lowering the gamma value from 1.00 down to 0.88:

To help make it easier to see just how much of an improvement we've been able to make with the three sliders in the Levels dialog box, here's a "before and after" view of the photo. The left half shows the original, unedited image, while the right half shows the adjusted version:

Now that we've seen how easy it is to correct overall image tone problems with Levels, let's see how we can make the exact same corrections and more using Photoshop's Curves command!

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