How to Understand the Curves Tool in Photoshop

After you’ve mastered Levels, it’s time to take a step up to the tool that is probably the most useful for color and contrast control in Photoshop: Curves. As with levels, you should play around with the basic Curves command to get a feel for it. For safety’s sake, begin by duplicating your background layer, either from the Layer>New Layer from Copy menu, or use the shortcut Control/Command + J. You should make a habit of learning shortcuts because it speeds up your workflow.

The Curves Dialog

Here’s our starting image for this tutorial, shot on a hazy day at Formentor in Mallorca.

Formentor, the northern tip of Mallorca

Use Control/Command + M to invoke the Curves dialog. Taking a look, you’ll see the main part of the dialog is the Curve itself. By default it’s a straight line from bottom left to top right – this is the baseline. There’s faded histogram in the background that corresponds to the tones in the image. There are two other tools to help you work in Curves.

There are two other tools to help you work in Curves. The first is the Channel Overlays, which show you the changes you make when applying curves to the individual red, green, and blue channel curves. The second is Intersection Lines, which appear when you drag a point on the curve. They let you see where the point is relative to the grid. The Channels Overlays, the Histogram, the Baseline, and the Intersection lines can all be turned off in the “Show” section of the dialog box. They’re all on by default.

Another default is that the darker tones are on the left, and the lighter tones are on the right. It can be swapped by clicking the “Show Amount of:” option from Light (0-255) to Pigment/Ink %. Unless you have a specific reason to change it, leave it on the default option Light. The final option is for the Grid. You can change from quarters to tenths by alternating the grid size icon. Alternatively, you can Alt or Option click directly on the Grid to toggle between them.

Starting with Curves

To change the Curve, you click on the baseline to create a control point (we’ll just call them points from here). By dragging the point up, you will lighten the image at those tones and surrounding tone. Drag down, and you’ll see the image darken at the tones around your point.

The most basic change you can make is to click on the center of the curve and drag up. This lightens the mid-tones and acts in a similar way to the Brightness slider, but with more control over the center-point of the effect. By the way, you don’t even need to drag the point. Instead, you can click to create a point and then use the Up/Down/Left/Right arrow keys to move it. A single arrow click moves 1 point (in the range 0-255), while holding the Shift key down as well, moves it 10 points. For utter precision, you can enter the number directly into the Input (that’s the point you click) and Output (that’s the place it moves to).

While it will work as a brightness tool, Curves is really about contrast. Contrast defines the relationship between the tones in your photo. A high contrast image will generally have strong shadows and highlights, with a lot of saturation. Low contrast photos will look flat and almost gray. That’s not to say low contrast is bad, it can be ethereal like the woods in fog, or a flared backlit portrait at sunset. A lot of photos can benefit from an increase in contrast.

So how can you do this? Well, you’ll need to make the shadows darker and the highlights lighter.

Creating Contrast

First of all, click on the line about a quarter of the way in from the left. Next, drag the point you’ve created down. This will darken your entire image. You don’t need to drag it down much to see a difference. After this, you should create a second point, this time about three-quarters of the way along and drag it up a little. Again, only a small amount is needed for this to work.

If you look at your curve, you’ll notice it resembles an S-shape. This S-Curve is probably one of the most common ways for you to create contrast with Curves. Our sample photo has better contrast now, but it is still a little dark, we’ll come to that shortly.

Notice how the line now resembles the shape of the letter S.

As another way of working, start by placing a point in the center. This will be your anchor point. You’re not going to move this one. Now create the point for your shadows and drag it down. You’ll notice that as you drag the shadows down, the highlights will move up automatically across from the anchor point. By clicking and dragging the center up, you both brighten the image and increase contrast. If you want to control the image more, just add more points.

Endpoints and Input

You’re not restricted to making just curves. You can also move the endpoints of the line in. This results in a look that’s similar to when you move the black and white points using Levels. This adjustment makes a huge difference with this image, where most of the tonal information is in the center of the histogram. Notice that as you move the endpoints in the black and white points on the Input Slider move to a corresponding position – so you could also just move these sliders.

Moving the endpoints in curves.

To finish up this look, you can add a little S-Curve after moving the end points. You’ll notice that the increased contrast has also led to increased color saturation.

Erase and Rewind

At any point that you want to start again without closing the dialog and reopening it, you can hold down the Alt or Option key, and click the Reset button that appears instead of the Cancel button.

If you only want to remove a single point, you’ve got a few options. Drag the point right off the grid. Or, you can click on a point and press the Delete key. The last option is to Control/Command+click on a point to remove it.

Presets

Just by way of mention, you should try the Curves presets in the menu to get a feel for what particular points can do to a curve. You can also save any curve you create yourself by clicking on the tiny cog beside the menu and choose “Save Preset”. Your Curves preset will then get added to the menu.

Auto and Options

Just like Levels, there’s an Auto function for Curves. It too depends on the Options section. Rather than repeat it verbatim, go check out the Auto section in the Levels article.

Using Curves as an Adjustment Layer

Up to now, what you’ve done has been destructive editing. You’re completely changing the layer to which you apply the Curve. Instead, it’s best to use Curves as an Adjustment layer. From the bottom of the Layers panel, click the split circle icon and choose Curves.

The Curve will appear in the Properties panel. Using the information you’ve got so far it should be no problem editing a file like this. I’ve started by pressing the Auto button, then added a curve to both increase contrast and brightness.

Sectioning your photo with Curves Adjustment layers

So far you’ve been using Curves on the entire image. Not all areas in a photo may need the same work, but it’s easy for you to select different areas to work on.

Press L to bring up the Lasso tool, or select it in the toolbar. If it’s not the main lasso, use Shift+L to cycle through the options. At the top, enter 15px in the Feather option. You’re going to select the sky first. In order to draw outside the photo edge, hold down the Alt/Option key. You need to do this so the Feather, which softens the edge of the selection, doesn’t soften the edge of the photo. Join the two ends of the lasso to make a selection.

Now, create a new Curves Adjustment Layer. Notice that the mask beside the curve is white on top, and black on the bottom? That means your curve will only affect the top of the image. White means the effect is revealed, whereas black means it’s concealed. You can use the brush tool to add to a mask or remove from it using a black brush. Shades of gray will work too but act like an opacity control.

You can now go and edit the curve for the sky, separately to the main curve. Another advantage to using Curves as an adjustment layer is you can overdo the look of the curve, and then bring it back using the layer’s opacity slider.

Before Curves adjustments.

Here’s the version after curves have been applied.

The Tip of the Iceberg

Curves is a pretty useful tool and more versatile than Levels for advanced contrast control. You’ve seen a lot of what can be done here, but there’s also the color changes that can be done using the individual color channel curves, but that’s a topic for another article.

The post How to Understand the Curves Tool in Photoshop by Sean McCormack appeared first on Digital Photography School.

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