I’ve already discussed some of your camera’s limitationsarticle. In another articlelink I discussed the three equally-important methods to control how much light your sensor collects and the side effects of each method. Today’s discussion is about how to use your camera’s semi-automatic exposure modes. We will briefly review some of the earlier principles but will move beyond the single basic assumption that most experts cover as we discuss the impact that metering modes have on your camera’s control of an image’s exposure.
Why Not Manual Mode?
Although Manual mode gives you the best control, there are times when lighting conditions change too fast. And sometimes it is just more convenient to use one of the advanced auto-exposure modes. This lets the camera set the exposure level by adjusting one of the three controls – shutter speed, aperture, or ISO – while you control the other two. For Canon cameras the mode is called Aperture-Priority if you set the aperture and ISO (and let the camera control shutter speed), Shutter-Priority if you set the shutter speed and ISO (letting the camera control aperture), and Manual Exposure with Auto ISO if you set the shutter speed and aperture (leaving the camera ISO). Never let the camera control more than one variable. Keep in mind, the camera now has TOTAL control to adjust the exposure; your two controls are only for the side effects – motion blur for shutter speed, depth of field for aperture, and noise for ISO. Although your two parameters always have an impact on exposure, the camera gets the last word and uses its parameter to overcome those impacts. If you want to have any say at all about exposure, you must use exposure compensation. But why would that be necessary?
What The Camera Considers
The main reason you need to use exposure compensation is that the camera programmer has no clue as to where you are pointing your camera or what kind of picture you are taking. So s/he is forced to make assumptions. (There is an old saying about when you ASSUME you make an ASS of U and ME. Unfortunately in this case, as an inanimate object the camera is incapable of accepting its share of the responsibility.) The most important assumption the camera makes is that your picture (subject) is of average light intensity. No matter what you shoot, the camera will let in enough light to make it a medium gray (or colors that would translate to medium gray with black-and-white film). Many times this is fine, but nobody likes gray snow in their winter vacation pictures or pictures of the rare gray bear from their trip to Yellowstone. To make your snow white again, you would need to apply a positive exposure compensation before taking your first shot to tell the camera to let in more light than it really wants to (for a brighter subject than it was expecting). To bring those black bears back to life, you would need to apply a negative compensation. Regardless of the subject, to tweak the exposure in subsequent shots, increase the exposure compensation to make the picture lighter, and decrease the compensation to make it darker. Once you’ve nailed it, you shouldn’t have to change the compensation on later pictures of the same subject, even if the lighting changes. You should be prepared to change the exposure compensation every time you change subjects, but not every time the light changes (that’s the only reason those three auto-exposure modes are so attractive).
And this is where most discussions of exposure compensation end.
What The Camera Can’t Consider (But You Must)
The camera can’t differentiate between your subject and its background. It further assumes that everything you might be interested in is at the same gray level. It’s fine if that happens to be true. But if they are not the same level, and if you go from a close up of the black bear (in an obviously lighter background) to a general landscape shot that includes the bear, then you might need to change the exposure compensation, or you might want to change your metering mode.
The metering mode determines how much of the view is considered in adjusting the exposure. On Nancy’s Canon EOS 7D, there are four modes. Spot metering, as you might guess, covers the smallest part of the center of the screen – about 2.3% of the viewfinder area. Next would be Partial metering, covering 9.4% of the total area. Then they get a little trickier. Center-weighted average metering gives an undisclosed, but presumably larger center area most of the weight, but does consider everything else in view. Finally, Evaluative metering, in which “the camera sets the exposure automatically [ed: read as “magically”] to suit the scene”. Right.
For consistent exposure compensation, the area considered should be smaller than the size of the subject if you want exposure compensation to behave as described in the “What The Camera Considers” section. If your backgrounds are more stable (whether or not they happen to be the equivalent of a medium gray), then a wider-looking metering mode might be better. Then you might be able to go from shooting a (black) crow to a (white) ibis as you hiked the consistently-lighted trail without having to adjust the exposure compensation at all. But this is starting to get into personal preference. Experiment with all of your metering modes and decide what works for you. Just be aware that if your settings or situation changes (as when you are metering on the background and the subject moves ‘too’ close, or when you are metering on the subject and then widen out for a cover shot), the computer may start behaving in the opposite manner than what you were expecting. Now you know why. It is (probably) just because you and your camera weren’t on the same page as to what was the important part of the picture.
I hope this helps you work with your camera instead of against it in your efforts to get the best shot. Good luck.