How To Use Histograms on your DSLR

Histograms are some of the most useful tools in digital cameras, but few know how to use them. Understanding the information that they provide will help you find the perfect exposure for your photos.

Be Skeptical

Why would you need yet another exposure suggestion when you have both your light meter and your camera's digital display? Though the light meter has become universally trusted as the primary guide to exposure, it only provides you with a little bit of information. Most in camera light meters measure the amount of light in the entire frame, and then suggest a setting that will average all the pixels into exposure. However, you may run into a situation where your subject is juxtaposed, with a background at an extreme end of the spectrum. For example, if the person you are photographing is standing against a white wall, your light meter will try to average all of the white pixels into the middle gray range. Therefore, your photo will end up being underexposed. 

You may compensate by trusting your display, which shows you what you have just photographed. However, this is not foolproof either. The display on digital cameras is relatively small, so the image is somewhat compressed and therefore not completely accurate. The screen's brightness settings and glare from ambient light can also throw off your perception.

Understand the Histogram

Set your camera to show you the histogram. It should appear to be a series of spikes across a horizontal plane. This is essentially a graph. The vertical axis represents the number of pixels, and the horizontal axis represents the level of light. The far left end of the graph corresponds to black pixels and the far right represents white pixels. The histogram tells you how many pixels of each light intensity are in your photo. A normal situation that is exposed properly should look like a round hump right in the middle of your histogram. If you see a pile up of pixels on the left side of the graph, then your photo is likely to be dark. If the stack of pixels is on the right side, then your photo may be overexposed.

Applying the Information

Watch your histogram as you change your aperture. As you open up, the entire graph should shift towards the right, and as you close down, it should drift towards the left. Usually, you want to avoid a build up of pixels on either extreme because those are generally dead pixels (they are not getting any information). However, be aware of what is in your photo.  In the example described in step 1, you have a subject against a white wall. In this histogram, you will have a big spike near the white side of the graph. You may want to pull them just out of the blown out section, but understand that these are meant to exist as white.

Histograms are the most reliable tools for judging exposure. Understanding them will keep you on track in all your ventures.

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