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Once you get your image file on your computer ready to edit (see the Essentials and Part I of this series), the Levels - Histogram tool should be your first stop in your image editing software. It can do a lot to correct exposure problems. My examples here are from Adode PhotoShop, but a Levels or Histogram tool should be available in other popular imaging software, such as JASC Paint Shop Pro, Microsoft Picture It! and others.
In Photoshop, click on "Image" on the menu bar, then "Adjustments" (or "Adjust") and "Levels..." Here’s what the Levels tool looks like on a rather monochromatic picture of an eagle ray (Figure 1), taken in Bonaire by the way. At the far left end of the histogram is an indication of how many pixels are the blackest black available (see the little black slider at the left 
), and it ranges to the mid-tones (see the little gray slider in the middle 
) to the whitest white pixels at the far right (see the little white slider at the right 
). The histogram here shows this picture doesn’t have a lot of real bright pixels and even fewer dark pixels. All the pixels are lumped at different brightness regions from not very dark blue to not very light blue:
Figure 1
The picture would be more pleasing with a nice spread across the entire range, spreading the contrast of the pixels from very dark to the lightest. So to do that, I put my mouse on the histogram and first grab the white point slider at the far right, and I drag it to the left until it just barely comes under the pixels indicated there (Figure 2). If I move it too far to the left I might start to lose detail by lightening too many pixels too much. You can check and uncheck the "Preview" box, to see what the picture will look like before and after.
Figure 2
Now I grab the black point slider on the far left, and drag it to the right until it barely comes under the first group of pixels on the right (Figure 3). If I drag it too far to the right I might turn too many pixels too dark and lose detail.
Figure 3
See how the last picture of the eagle ray there in Figure 3 has better dark-to-highlight contrast than Figure 1? Click the OK button to apply the changes.
Here a couple of useful tips: First, when using most any tool's dialog box in Photoshop, you can reset the tool to all it's default values as it appeared when first opened. Holding down the Alt key in Windows (Option key on a Macintosh) changes the Cancel button to Reset, and then click on the Reset button.
Second, there is some specific help with the Levels tool. When I hold down the Alt key in Windows (Option on Mac) and then click the white point slider at the far right, the entire preview image goes black except for the brightest pixels above and to the right of the slider in the histogram. Figure 4 shows the preview image with the Alt key (Option on Mac) pressed while I drag the white point slider to the left; the blue pixels in the lower left of the preview image are the brightest pixels in the image, where the white sand is closest to my strobe light. With the Alt key (Option on Mac) still depressed, I move the slider back a little to the right, so that the slider is just under where only a few of the brightest pixels are indicated on the right end of the histogram, so almost all of the image previews black with just a few brighter speckles. Then I let go of the Alt key (Option on Mac) and the preview window returns to normal with the white point set a little more precisely.
Figure 4
The Alt key in Windows (Option on Mac) works similarly for the black point slider on the left end (Figure 5). With the Alt key (Option on Mac) pressed, I drag the black point slider to the right, and the entire preview image goes white except for the darkest pixels in the upper right of the image, indicated in the histogram above and to the left of the slider. With the Alt key (Option on Mac) still depressed, I move the slider back a little to the left, so that the slider is just under where only a few dark pixels appear on the left end of the histogram, so almost all of the image previews white with just a few that are darker. Then I let go of the Alt key (Option on Mac) and the preview window returns to normal with the black point set.
Figure 5
When you are correcting exposure, it’s better to start with an image that’s a little too dark rather than too light. You can bring some detail out of shadows, but you can’t get anything out of overexposed highlights – white is only white.
Here’s another example, this time lightening the mid-tones only, using just the gray slider in the center:
Figure 6
As you can see by the big black lump on the left side of the histogram in Figure 6, this image has a lot of dark to mid-tones, but fewer mid-tone to light pixels. It does have some very light and white pixels at the right end, but there are quite a few more on the dark (left) side. If I started by moving the white point slider at the right end of the histogram to the left as I did in the above example, I would start to blow out some highlights. Instead, I start with the gray slider in the center and move it to the left to brighten the mid-tones:
Figure 7
I hope you can see a difference in this second image of the two butterflyfish. It’s a subtle difference that may not translate well here, but the mid-tones are generally brighter. Come on over to my house and I’ll show you.
There are other ways to use the Levels tool. For example, you can use the eyedroppers in the lower right to sample and set the black, mid-tone and white points. You can work on Red, Green and Blue color channels individually rather than than all three at once (see the box in the figures, "Channel: RGB"), although I prefer to use Levels for exposure and do color adjustments with the Color Balance and Hue/Saturation tools. So experiment with it. In general, take small steps in your image editing starting with the Levels tool. Large changes may tend to pile up and result in unnatural looking images. Next we’ll have the exciting and lovely Curves tool.
Previous: Digital Imaging Editing - Part I
Next: Digital Imaging Editing - Part III
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