Before we begin our look at Curves, I'm going to reset my image back to its original state by temporarily turning off my Levels adjustment layer. To do that, I simply need to click on the Layer Visibility icon (the "eyeball" icon) on the far left of the adjustment layer in the Layers palette:

With the effects of the Levels adjustment now hidden, my image returns to its original, unedited state:

To access the Curves adjustment, I'll once again click on the New Adjustment Layer icon at the bottom of the Layers palette, and this time, I'll select Curves from the list of adjustment layers that appears:

This brings up the Curves dialog box. Again, I'm using Photoshop CS2 here since we're only looking at the basics of how Curves works. The Curves dialog box has a few additional features in Photoshop CS3 and higher which we'll save for another tutorial.

The Levels and Curves commands are actually very similar in that they both allow us to adjust the shadows, highlights and midtones of an image, but you'd be forgiven for not seeing any similarity between them just by looking at their dialog boxes. Levels seems fairly straightforward with its histogram, gradient bar and sliders, whereas Curves looks like something you'd find in a science lab. It may be called Curves, but where are they? There are no curves to be found anywhere! Instead, we see a 4x4 grid with a diagonal line running through it from the bottom left corner to the top right corner. The only similarity with Levels seems to be that there is a black-to-white gradient bar running along the bottom of the grid, but there's also a second gradient bar running up and down along the left side of the grid:

With no curves to be found, why is it called Curves? The reason has nothing to do with what you start with and everything to do with what you end up with. Curves is all about taking that straight diagonal line running through the grid and reshaping it into a curve! At first, the line is straight because we haven't yet made any changes. As we bend the line to create a curve (or curves), we make adjustments to the various brightness levels in the image. Before we get ahead of ourselves, let's first look at what it is we're actually seeing in the Curves dialog box and how similar it really is to Levels.

Levels vs Curves - The Histogram

One of the most obvious differences between the Levels and Curves dialog boxes, at least in Photoshop CS2 and earlier, is that Curves does not contain a histogram. In Photoshop CS3 and higher, Adobe did add the option to view the histogram inside the Curves grid, but I still find that the best way to view the histogram while working on an image is with the Histogram palette, since it gives you an updated view of the histogram as you're working. In both Levels and Curves (in Photoshop CS3 and higher), the histogram remains static, showing you only what the image looked like before making your adjustments, so you're really not missing all that much without the histogram in Curves as long as you keep your Histogram palette open as you're working. There are a couple of benefits to having the histogram displayed in the Curves grid, but nothing we need to worry about here.

Levels vs Curves - The Gradient Bar

The Curves dialog box contains the same black-to-white horizontal gradient bar below the grid that the Levels dialog box shows us below the histogram. In both cases. this gradient represents the various brightness levels in the image from pure black to pure white. With Curves, we get a second gradient bar as well, this time running vertically along the left side of the grid. The difference between the two gradient bars in Curves is that the bottom one shows us the Input levels while the gradient along the left shows us the Output levels. Think of "Input" and "Output" as "Before" and "After". The bottom gradient represents the original brightness levels in the image before making any changes, while the left gradient shows us what the new brightness levels will be after making the adjustments. We'll see how this works in a moment:

Levels vs Curves - The Black And White Point Sliders

In Levels, we had a black point slider below the left side of the histogram and a white point slider below the right side of the histogram. These sliders allowed us to set new black and white points in the image, darkening the shadows and brightening the highlights. If you look closely at the diagonal line in Curves, you'll see a small square on either end, one in the bottom left corner directly above pure black in the gradient below it and one in the top right corner directly above pure white in the gradient. These squares, or points, are the Curves equivalent of the black and white sliders in the Levels dialog box. We can click on either point to select it, and by dragging the point either left or right, we can set new black and white points for the image:

Let's see if we can use what we've learned so far about Curves to make the same basic tonal adjustments to the image that we made with Levels. To darken the shadow areas with Levels and set a new black point, I clicked on the black point slider and dragged it in to the left edge of my histogram. If you recall, this raised the black point from a default brightness level of 0 to level 20, which meant that any pixels that were originally at a brightness level of 20 were pulled down to pure black. We can do the exact same thing with Curves. To set a new black point, I simply need to click on the point on the left end of the diagonal line and drag it towards the right, just as I did with the black point slider in Levels. Since I'm using Photoshop CS2 and don't have a histogram displayed in the Curves dialog box, I'll keep an eye on the Histogram palette to see what's happening.

Notice that as soon as you click on the point, both the Input and Output readings below the bottom gradient bar show a value of 0, letting us know that the black point is currently set to a brightness value of 0 and we haven't yet made any changes. As you drag the point horizontally towards the right, the Input reading increases while the Output reading remains at 0. This tells us that whichever original brightness level we select with the point (the Input level) is being pulled down to level 0, or pure black (the Output level). I'm going to drag my black point to level 20, the same value I set it to in Levels:

To set a new white point in Levels and brighten the highlights, I dragged the white point slider in to the right edge of the histogram, lowering the white point from it's original value of 255 down to 232. This pushed any areas in the image that were originally at a brightness level of 232 up to pure white. Once again, I can do the exact same thing with Curves. The point on the right end of the diagonal line works exactly the same way as the white point slider in Levels. I simply need to click on the point and drag it horizontally towards the left. Again, the Curves dialog box in Photoshop CS2 does not contain a histogram, but I can easily view the histogram in the Histogram palette as I drag the point.

As soon as you click on the point in the top right corner, you'll see the Input and Output readings display a value of 255, indicating that our white point is currently set to its maximum brightness value of 255 and we haven't yet made any adjustments. As you drag the point towards the left, the Input reading changes while the Output reading remains at 255, telling us that whichever brightness level we select with the point is being pushed up to pure white. I'm going to drag the point towards the left until the Input reading is displaying a value of 232, the same value I set the white point to in Levels:

If I look at my Histogram palette once again, I can see that I've successfully made the exact same adjustments to the image with Curves that I made with the black and white point sliders in Levels. The histogram now extends all the way from the far left to the far right, showing a full range of tonal values from pure black to pure white, except of course for the detail we've lost by stretching the tonal range, indicated by the same comb-like pattern we saw earlier:

And if we look at the image itself in the document window, we can see that its overall tone has been improved:

What about the midtone slider? We'll look at that next!