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There is a certain mystique about the Curves control in Photoshop. If you have mastered Curves, you are looked upon as a Photoshop guru. If you have only dabbled with Curves, you have likely been intimidated by it, and decided to stick with Levels or even Brightness & Contrast for your tonal adjustments.

Curves is an incredibly powerful tool in Photoshop, which allows you to exercise tremendous control over your tonal (and even color) adjustments. That power is accompanied by complexity and a steep learning curve (pun very much intended). However, the Curves control isn’t as challenging as it first appears, and with some practice it can be mastered by all photographers using the digital darkroom to optimize their images.

When looking at the Curves dialog box, you can interpret how the image is being adjusted for any tonal value. Simply look at the bottom gradient for the tonal value you want to translate, and then moving up vertically until you reach the curve. At that point, move directly to the left to determine which tonal value the original will be changed to.

Once you understand how the “before” and “after” values work with the Curves dialog box, you can start to understand how to make adjustments to your image using Curves.

To get started, create a new adjustment layer from Curves so you can configure the dialog box to your liking. In the bottom-right corner of the Curves dialog box is a button to increase or decrease the size of the dialog box. You can have the dialog “large” or “small”, and the button will toggle the size accordingly. I would recommend always working with the larger size if your monitor resolution allows.

The next step is to ensure the grid is helping you break down the tonal values into manageable sections. I prefer to break the tonal scale of the grid into tenths rather than quarters. To toggle between the two, hold the Alt key (Option key on Macintosh) and click in the grid area. Keep in mind that this grid is only for your reference, and doesn’t affect the actual adjustments to your image.

The gradients that define the “before” and “after” values in the Curves dialog box can also be toggled depending on how you prefer to think about your images. If you generally think in terms of ink on paper (which is my preference), you’ll want to have black to the right and top of each axis. That way, when you want to increase the amount of ink on paper (darken a tonal value) you move the curve upward to increase ink. If, on the other hand, you prefer to think about light, you can have white to the right and top of each axis. Then, moving the curve up coincides with increasing the amount of light, or lightening the given tonal value. To switch between these options, click your mouse anywhere on the bottom gradient.

Before starting to work on your image with the Curves control, you’ll want to have an idea of what you’re trying to accomplish. The Curves dialog box doesn’t really lend itself to “playing around” with your image. You’ll want to know what areas need lightening, and which need darkening, and then take steps to put those ideas into action.

When you know what area of your image you want to adjust, you’ll need to know what part of the curve relates to that area. Fortunately, this is easy to accomplish. Simply click and drag your image over the area of the image you want to adjust. A “bouncing ball” will show up on the curve line, showing you where on the curve the tonal values you are dragging over correspond.
 
Once you know where on the curve you want to adjust, you can create anchor points so you can change the shape of the curve in that location. Simply click on the location you want to edit, and then drag the newly created anchor point up or down to adjust the tonal values represented by that portion of the curve. Note that when you have several anchor points, the black one is the currently selected point. You can use your mouse to move the anchor point, but you can also use the arrow keys on your keyboard to move the selected anchor point.

I’ll often actually create three anchor points when I really only need one. Between these two I’ll place an anchor point that I’ll actually use to adjust tonal values. The outer anchor points simply define the outer limits of the tonal range I want to adjust.

When you make adjustments using curves, you will generally affect the whole curve, not just the area you created an anchor point for. To correct for this, create new anchor points outside the area you were adjusting, and drag them to a location that allows the rest of the curve to return to “normal”. When I’m finished, I can add anchor points outside my original two, placing them very close to the anchor points that define the outer edges of the tonal range I’m adjusting, so that the rest of the curve will be virtually unaffected by the changes I made.

If you create an anchor point you don’t want, it is easy to get rid of it. Simply click on the anchor point, and then drag it off the Curves grid. Once outside the grid area you can release the mouse and the anchor point will be removed.

As you are adjusting the curve for an image, note that areas that are steeper than the original 45-degree line represent an increase in contrast. Areas on the curve that are shallower than the original 45-degree line represent a reduction in contrast. Therefore, you can increase or decrease contrast to different degrees for a variety of tonal values in your image.

One common cause for alarm with Curves is that it can have a dramatic impact on the hue and saturation of the colors in your image. If your Curves adjustments are causing these problems, simply set the blending mode for the Curves adjustment layer to “Luminosity” rather than the default of “Normal”. This will cause the Curves adjustment layer to only affect brightness and contrast, with no change in color values.

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I am a beginner in photoshop. After going through your article on curves, many of my doubts have been clarified. Your article is ver informative. Thank's a lot for publishing such a nice article in simple language which a lay man like me can understand. Thank you very much. Posted by: LAKSHMANSASTRIMAHAPRASAD Oct 15, 2006 @ 6:17 AM EST

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