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After using the Levels - Histogram tool to adjust overall exposure (see Part II), you might look at your image and decide you want to fine tune exposure a little more. Say you have a nice black or blue water background behind your primary subject, and you want to brighten up the subject’s highlights without messing with the generally dark background. The Curves tool in Photoshop lets you modify exposure and contrast affecting certain brightness/darkness ranges more than others. You can focus on certain ranges specifically while leaving others alone.
By the way, do not touch the Brightness/Contrast tool! (Put down the pliers, Eugene!) The Brightness slider in that tool apparently changes every pixel’s brightness by the same amount - it’s a global change. So it screws up that nice exposure adjustment you just made with Levels (previous article). And I don’t know for sure what the Contrast slider is doing, but I don’t like it. Levels and Curves give you a lot more control, so I recommend you leave the Brightness/Contrast tool alone.
In Photoshop, click on "Image" on the menu bar, then "Adjustments" (or "Adjust") and "Curves..." Here’s what the Curves tool dialog box looks like when you first open it (Figure 1). The diagonal line in the graph going from the bottom left (darkest levels) to the top right (brightest) is the curve that you will manipulate. It maps Input values along the horizontal X-axis to Output values along the vertical Y-axis. You might think of Input as pixel values before any changes and Output as afterwards. When you first open the tool the curve is a straight line from corner to corner, indicating no changes yet - input equals output.
Figure 1
Here are several versions of a picture of an angelfish, examples to introduce you to what Curves can do. The first image (Figure 2) is with Curves open with no changes yet. I clicked the mouse once in the center to add a point - notice that the Input and Output values are the same at that point, 128:
Figure 2
For the next image I clicked on the center point and dragged it to the left (Figure 3):
Figure 3
And for the next I moved the center point by the same amount but I moved it up instead of left (Figure 4). Notice the shape of the curve for these two is almost the same, so the changes to the picture are almost the same - the black point and white points are left alone but the mid-tones are lightened considerably, with the most brightening in the exact center of the brightness range:
Figure 4
Now, if I wanted to make the darker pixels a little darker and lighten up the brighter pixels - increase overall contrast - I could drop two points and make an S curve: Click in the lower left quadrant and drag the curve down and to the right, and click in the upper right and drag the curve up and to the left (Figure 5):
Figure 5
You can click on the curve anywhere and put more than one point down, and you can then drag the points around one at a time. Or you can select multiple points to drag around by clicking on points while holding down the Shift key. The points that you don’t move act as anchor points and keep the brightness levels in their area from changing. For example, if I wanted to brighten up the face and yellow flecks on the angelfish but leave the blue water background and darker parts of the fish alone - brighten the upper midtones only - I could click and put down four anchor points in the lower left quadrant and then drag a point in the upper right quadrant up and to the left (Figure 6). It’s a subtle difference, but compared to the original image on the left, in the image on the right the fish’s face and other upper midrange pixels are brightened while the darker pixels are left alone:
Figure 6
I could take up a lot of space with Curves examples, but it’s a lot easier to do than read about. You can do some very subtle changes to only certain pixels, and you can also do some really bizarre tweaks. Try it out. Next we’ll look at the Color Balance tool.
Previous: Digital Imaging Editing - Part II
Next: Digital Imaging Editing - Part IV
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