Exposure Compensation

You may have noticed a button or menu item that provides exposure compensation on your digital (or for that matter, film) camera.  What does it do & how do you use it to improve your photographs?

When the camera mode is set to Program, or Aperture or Shutter Priority, you are using the metering system built into the camera to determine exposure.  Most of the time this works, however there are situations where the metering system can be fooled.  An example – take a photograph of a white dog against a field of snow.  Everything is white however the camera’s metering system will attempt to set the exposure as if the scene contained the average range from dark to light – the results is a gray dog.  The reason? Most metering systems assume the scene you are photographing contains a normal range of brightness. If you are photographing either extreme, your meter will likely provide incorrect settings.  In the case of the white dog against show, the metering system tries to average it, turning everything to gray. This is where exposure compensation becomes useful.

It is always worth letting the camera metering choose the settings at first.  Modern cameras, both point & shoot & DSLRs, particularly in the program mode are capable of finding the correct exposures in many difficult lighting conditions.  Still, if the image is important to you, it is a good idea to check that the exposure is correct. The easiest way to do this is check the histogram for the image. A review of histograms, what they are & how to read them is available in my previous article on Exposure.

An ideal histogram tapers to the base line prior to either the left or right edge of the window. If it extends off either end of the range above the base line, there is an exposure problem.  If it extends off the right, the image is overexposed, the left, underexposed.

You do need to a little interpretation here – The right end of the histogram is pure white.  Anything above that is recorded as white or clear, with no detail. On the opposite end, anything below the left end is pure black, again recorded with no detail.  If you are taking photographs of fireworks, lightning, city lights against a black background, you can expect the data to be heavily weighted to the left.  If you are photographing clouds, there may be some data to the right.  Here you need to take care.  Burned out skies can quickly destroy a otherwise beautiful image. Generally, if the range of light exceeds the range of the sensor, it is better to let it go over on the left, the dark portion of the image, because the missing information is buried in the shadows. Even small amounts of burn out caused be exceeding the capability of the sensor at the bright (right) end is obvious.

Another problem, particularly at the right side of the histogram, is color shifts.  Although a few cameras can show the histogram for each of the primary colors, most cameras show a histogram that is an average of the red, blue & green data.  If the scene is not evenly balanced between the three colors, one may go over before the others.  Since data that exceeds the upper limit is not recorded, the other colors will appear stronger than they should be. This can be a real problem if you are shooting under colored light, or monochrome scenery.

If your image is shifted too far to the left, it will be too dark. you may be able to bring it back up, however using an editor to brighten an image will increase the noise levels making the image grainy. The ideal exposure is as far right as possible, as long as no color goes off scale.

If it appears to be running off either end, using exposure compensation can produce better images.  As you move the compensation in the “+” direction, the amount of light striking the sensor will increase.  As you move it towards the “-“, less light strikes the sensor. The amount of adjustment per step is usually set with a menu.  Typically it is in 1/3, 1/2 or 1 full stops.

By the way, if your camera does not have an exposure compensation button or menu, as long as it has a manual mode you can still make corrections. Check what shutter speed & aperture the camera chose, switch the camera to manual, set the shutter speed & aperture to the same, then reset either the aperture or shutter speed to make the same correction you would do using the exposure compensation button – Increase the aperture or choose a slower shutter speed to add more light, the opposite to decrease the amount of light.

Don’t confuse flash compensation with exposure compensation. Some cameras provide both. It does much the same thing, but rather than controlling overall exposure it adjusts the amount of light produced by your flash.  This can be very useful if you are trying to use fill flash mixed with natural light – if the flash is too prominent producing harsh shadows, use the flash compensation to drop it down a bit & reshoot. Highly reflective rooms can cause overall overexposure problems. Any time your flash seems to be overpowering the image, flash compensation can be used to solve the problem. Although you can use your histogram to determine if the flash is burning out the image, you need to look at the LCD to determine the balance between natural light & the flash fill.  In some cases, the LCD may not show enough detail – you may need to view the image on a laptop or other large, high resolution screen to check the balance.

One very important point – after using exposure or flash compensation, be sure to set them back to “0”. If you don’t, I guarentee the next image you take won’t need the correction, will be a chance in a lifetime capture, and you will forget (or won’t have time) to check the quality of the image until it is too late to do it over!