Using Your Camera's Histogram

by Sue Barthelow

Want to get the right exposure for your digital camera pictures? Try using your camera's histogram display to view your photo's tonal range. The histogram will indicate whether you should adjust your settings and take another shot. Check your camera's manual to find out how to view a histogram and how to change the camera's settings.

The histogram shows you a graph of your photo's range of light. The horizontal axis represents the brightness values, going from black on the left (brightness value of 0) to white on the right (brightness value of 255). The vertical axis shows the number of pixels in the photo that have the corresponding brightness values (0 to 255).

For ideal photos taken in the best lighting conditions, the high points in the graph are near the center. The heights fall in both directions, extending to the left and right edges while not running off the graph's edge. A graph like this indicates that the photo's exposure is optimal, with great midtones and a range of light that captures everything from black to white.

A graph that has brightness values running off the left edge or that has its high points well left of the center indicates that the photo is underexposed. You'll get a better photo by taking another shot after adjusting your camera settings to let more light in. You can do any of the following: 1) Lower the f-stop to give you a larger diaphragm. 2) Slow down your shutter speed to keep the shutter open longer. 3) Raise your ISO to make the sensor more sensitive to the light. If your exposure is only slightly off, you can lower the f-stop by increments of less than a full stop using exposure value compensation.

A graph that has brightness values that run off the right edge or one that has its high points right of the center indicates that the photo is overexposed. In this case, you'll get a better photo by adjusting your camera settings to keep some of the light out. You can do any of the following: 1) Raise the f-stop to give you a smaller diaphragm. 2) Raise your shutter speed to decrease the amount of light that enters the lens. 3) Lower your ISO to make the sensor less sensitive to the light. If your exposure is only slightly off, you can raise the f-stop by increments of less than a full stop using exposure value compensation.

Note: The changes mentioned above are not the only things you can do to affect a photo's exposure. Your selected metering mode (matrix, center-weighted, spot) and the area you meter on both affect the exposure information. Most filters lower the amount of light that passes through the lens and help when you have an overly bright scene. When you have an overly dark scene, you can use a flash. Lastly and possibly the most important for outdoor photos, the time of day drastically affects the available light. Check your histogram to see how any changes you make affect your photo's exposure.