I see a lot of posts and questions from people that are getting poor exposures. In many cases the user tries to point the finger at the camera or lens somehow being defective. Most of the time this is not the case and it’s just an issue of understanding how your camera is metering and the differences between the metering modes.
Some users will settle into one metering mode and learn to work with it, which is fine. Some don’t even realize that different metering modes will yield different results and they just rely on evaluative metering to do the job. The reality is that all too often the camera does not make the correct decisions about exposure because of the metering mode used and lack of understanding how it’s working.
The meter is easily fooled by high contrast areas and an uneven proportion of highlight to shadow. When there is an even distribution of highlight and shadow the meter doesn’t get fooled and it’s able to correctly calculate the correct exposure (or close to it).
You can manually fool the meter by metering off a particular area of the scene, using exposure lock and then recomposing and shooting with that exposure.
You can also use exposure compensation to override the meter and adjust exposure for a particular area or to prevent a blown out sky or shadows that are in black. That can take some practice and experience to know how far to adjust and in what direction, plus or minus.
As a means of just showing a very basic example of how this works I’ve attached 3 images that were taken using evaluative metering. They were shot in Aperture Priority (Av) mode at f/8. I intentionally used a scene that had a mixture of highlight and shadow with significant contrast.
The first image was shot as you would normally take a photograph. I’ve composed the shot as I want, metered and took the photo. In the second shot I pointed the camera up so the entire center of the frame was on the sky, metered and locked exposure, recomposed and shot. Finally, the last shot was taken by pointing the camera down to the wooden deck, metering and locking exposure, recomposing and shooting. No exposure compensation was applied to any of the images and you can see how the metering and exposure are affected.
The camera determined that for the first exposure 1/100s was the appropriate shutter speed and for the most part did a good job, giving me good exposure on the sky, the trees in the middle and the wooden deck. In the second image that was exposed for the sky, I got a beautiful rich blue sky but the trees and wooden deck are slightly on the dark side. The camera determined that 1/200s (1 full stop less than standard exposure) was correct for the sky to be properly exposed. There is nothing wrong with that exposure and some might even like it more because of the saturated colors, and even though the trees and deck are dark there is no real loss of detail and it looks good. The third image, on the other hand, although exposed properly for the wooden deck, is now overexposing the sky and trees to the point where it is no longer pleasing or correct. For that shot, it determined that 1/60s (2/3 stop more than standard exposure) was correct to properly expose the wooden deck.
This is where the concept of creative exposure meets correct exposure. If you wanted to take control of the exposure and get the best of what the first and second exposure offer then you could dial in some negative EC, probably -1/3 or -2/3 stop, but not going down as far as the second image, which is 1 full stop down. Knowing this helps to figure out when to use exposure compensation and gives you a feel for how much just by being observant of the camera's 'suggested' exposures when it sees different amounts of shadow, highlight and contrast.