January/February 2008 HelpLine
• Inadequate Compensation
• Learning To Polarize
• Is It Stabilized?
As I start this column, the first of the new year, I'd like to thank all of you who have submitted questions to HelpLine through the years—they have helped make this column a useful resource for our readers. Keep them coming!
I'm trying to figure out how the exposure compensation works on my digital SLR. It doesn't seem to affect the metering. Am I using it the right way, or is there a problem with my camera?
Via the Internet
A) Exposure compensation doesn't affect your camera's metering. The light meter built into your camera is going to do one thing: measure the light coming into the camera. You can change how the meter reads the light by adjusting the metering method. Some cameras offer a center-weighted metering system that averages the entire scene but, as the name indicates, places emphasis on the center of the image. Some offer a spot-metering mode that allows you to pinpoint objects in the scene to measure. Most cameras default to a sophisticated metering system that takes into account the subject on which you're focused.
But none of this has anything to do with exposure compensation. Exposure compensation adjusts how the camera reacts to the meter reading. If you think that the camera meter reading is correct, then you leave exposure compensation alone.
Look at a test shot. If you think the image is underexposed, then you want the camera to "open up" the exposure a bit to let in more light. This could be done by a slower shutter speed or larger aperture. Underexposed images should be compensated for by a positive number and overexposed images by a negative number. These values actually represent exposure values (EV), but are more easily thought of in terms of "stops." An increase of one stop would be like going from a 1⁄250 sec. shutter speed to 1⁄125 sec. For aperture, it would be the same as going from ƒ/11 to ƒ/8.
Once again, when you adjust exposure compensation, you aren't changing the metering. You're telling the camera to react differently to the meter reading. If you're in aperture priority, the camera changes the shutter speed to adjust the amount of light that reaches the sensor during the exposure. Conversely, when you're in shutter priority, the camera adjusts the aperture. If your camera's program or auto mode allows for exposure compensation adjustment, whether the shutter speed or aperture (or both) is adjusted by exposure compensation is dependent on the camera. But when you're in manual mode, exposure compensation doesn't fit into the exposure equation—you're already explicitly setting both the shutter speed and the aperture.