09 March 2011

Exposure Compensation Tutorial

Exposure Compensation is the practice of adjusting for any kind of factors that might cause a camera or light meter to indicate a reading that gives an undesired result in the recorded image. If you are unfamiliar with the concept of exposure try reading the photography crash course. No piece of equipment is perfect. While modern cameras do a great job of measuring light and figuring out what the correct exposure is for a scene, they do make mistakes. Every photographer is bound to encounter difficult lighting environments that will fool his equipment. A classic example is a bride's dress and a groom's tuxedo:

The bride is wearing a white dress. All this white material is reflecting a ton of light back at the camera, which is fooled into thinking that the scene is extremely bright. Based on that reading the camera suggests settings that result in a very dark, underexposed image.
The groom is wearing a black tuxedo, black tie and black shoes. He's standing in front of a black limo. The camera sees all this dark material and thinks there's a lack of light, when in reality it's noon on a clear day. To make up for this perceived darkness the camera tries to make the image entirely too bright.

Exposure Compensation is a technique for correcting these situations. If the camera is being fooled into thinking the scene is too bright (the bride example), you apply negative exposure compensation. If it's incorrectly reading a scene as being too dark (the groom example), you apply positive exposure compensation.

Exposure Value

Most cameras have an exposure value (EV) meter similar to the one shown here. Exposure Value meter

In the middle is 0, the default setting with no compensation. To the left is two stops of underexposure and to the right is two stops of overexposure. This meter might be shown on the top display, inside the viewfinder, on the LCD or buried underneath menus. Like this example, most entry level, consumer and prosumer cameras have a four stop range. Pro cameras may have a wider range, for example Canon's EOS 1 series have a six stop range from -3 to +3.

Automatic Exposure Compensation

You can dial in exposure compensation when using automatic and semi-automatic modes such as Auto, Program and Aperture Priority. The camera will automatically apply your setting each time you hit the shutter button. If I were photographing the bride from our hypothetical situation in Program mode I might try adjusting the EV two notches to the left to -2/3. This tells the camera "underexpose the scene by two thirds of a stop" resulting in a slightly darker image than what the camera originally wanted to record. Each camera has a slightly different way of dialing in exposure compensation. Some have a dedicated button. Others require a combination of button presses. Read your camera's manual to find out how to adjust your EV meter. And don't forget to reset the EV to zero when you're done and move to a different lighting environment, otherwise you might go home with an entire day's photos wrongly exposed.

Manual Exposure Compensation

When shooting in manual mode, there is no need to adjust the EV meter. You apply exposure compensation simply by taking note of your camera/light meter's suggested exposure and choosing settings that either under or overexpose that reading. If I were photographing the groom from our hypothetical situation, I might do the following:

Choose the lowest ISO I think I can get away with given the available light.
Choose an aperture that gives my desired depth of field.
Frame up the shot and turn the shutter speed dial while watching the in-viewfinder EV meter, until it indicates +2/3.
Shoot the frame, then check the LCD/histogram/blinkies/whatever and decide whether the exposure is good, or I want to shoot again with more or less compensation.

Conclusion

Why did I pick +/- two thirds in each of these examples? It's a small number. Neither example is an extreme lighting situation, so 2/3 is just a slight adjustment from zero... not too dark, not too bright. If we were talking about an extreme range of light such as a silhouetted person backlit by bright sunlight then we would need bigger numbers like +2. How should you pick these numbers? Practice. Shoot thousands of photos and study the results, and you'll be able to just look at the available light and imagine what a stop or two more or less would look like. At this point you'll also know your equipment very well, to the point where you can anticipate when your camera will over or underexpose. Here are a few shortcuts you can remember:

In bright situations where the camera tries to make the image too dark, compensate to the right for a brighter exposure.
In dark situations where the camera tries to make the image too bright, compensate to the left for a darker exposure.
When the camera is tricked by bright materials/objects, compensate to the left for a darker exposure.
When the camera is tricked by dark materials/objects, compensate to the right for a brighter exposure.
The further you compensate to the left, the darker the image will be.
The further you compensate to the right, the brighter the image will be.

Exposure Compensation is Just One Technique

This is an exposure compensation tutorial, but I want to mention it's not the only way to get a good exposure. All of this assumes you're using a metering mode that takes the whole (or a majority) of the frame into consideration. A different method of handling these situations would be to take a light reading from a small portion of the frame which you know will have a good result. For example, you might switch your camera's metering mode to "Spot" metering and take a reading off the subject's face. In manual mode you can choose all your settings while reading that one spot. In a semi-auto mode you can press the exposure lock button to lock in that reading. Then you can recompose the frame and shoot the photo. Instead of compensating for the difficult factors (dark suit/bright dress), you've simply ignored them.