Pixiq

My students hear this all the time: The single most powerful tool for current digital imaging is the histogram, whether in the camera or in the Levels dialog of Photoshop.

A histogram is a graph of the tonal values of the pixels in an image—it plots out the actual tonal values of the photograph the moment I take the picture. The histogram in Photoshop’s Levels dialog displays exactly where tones fall, and with a little experience it becomes easy to deduce how they will print. The trick is understanding the histogram and learning to use it correctly.

Look at this image and the resulting histogram. The tones in the image range from medium-dark gray to medium-light gray; the histogram shows that very clearly.

None of my tones map out to the edges of the graph. Nothing hits the far left (pure black), and nothing hits the far right (pure white). As with any other scene, my camera meter tries to average everything out to neutral gray—something my Dad used to call trying to “see everything as gray.” It measures the entire scene and places the range of values from light to dark smack in the middle, giving me this histogram that sits in the center of the graph. I can photograph a black dog and my camera will try to make her gray. A snowy field—gray.

This is a great example. Look at the thin lines to the left and right of the main black area of the graph. They are almost perfectly centered between the pure black and pure white extremes.

If you let it, this image will print just as you see it—gray. You can adjust it in Photoshop, but the better thing to do is to capture it where you want it in the first place, mapping the whites to the white values and the grays to the gray values. That is our question—how do you tell the camera where you want to put these tones?

The answer is with the Exposure Value (EV+/-) control. Assume you want to shoot with automatic exposure, either in Shutter Priority or Aperture Priority. The EV control gives you the power to tell the camera to over- or under-expose the image. Going to the “+” side makes my snowy scene brighter; it pushes the tones up on the histogram and brings the lightest gray values closer to white. A little practice will show you how much to boost it, but the histogram on the camera will show you precisely what the effect of the boost will be.

Here’s what it looks like when I boost the exposure by +1. Immediately, you can see the snow reads white, but more importantly I can see on my histogram exactly how white that is. It is just a small skip away from the pure white of the far right of my graph, indicating that it still has tones. This is good; it means the file is not “blown out,” or losing digital information in the highlights. (Hint: Use the Highlight Alert function on your camera to indicate overexposed highlights.)

Envision your final image, use the EV+/- control, watch the histogram to get the most information from the capture, and ultimately create the highest quality photograph.