The importance of watching your exposure
One of the things I've learned over the time that I've been doing photography is the importance of watching my exposure. Not in the usual sense of noticing when the camera is putting you at a too low or absurdly high shutter speed (or ISO, or aperture), although that's important too. What I mean is watching your exposure from shot to shot.
Modern cameras in matrix or evaluative metering can and do change how they decide to expose a scene depending on just how it looks to them, regardless of whether or not the actual light has changed. Or in other words, if you change your composition the camera can change to a bad exposure. If your first photograph of a mixed scene with the clear sky visible has the sky correctly exposed at 1/200th at f/11 at ISO 200, you then recompose to show more of the shadowed ground, and the camera suddenly wants to be at 1/100th at f/8 at ISO 200, you are probably going to completely blow out the sky if you just automatically click the shutter.
(As they say, I've been there and done that.)
The same thing is true in more extended circumstances. If you're strolling along a path in a park taking pictures of different things, your exposure often should be fairly constant regardless of how much of the sky or sunlit ground is visible in each photograph. And if the camera's metering seems very off, you probably have a choice you need to make; you can let the camera more or less expose for the darker areas you're looking at (and accept that the bright areas may well blow out), expose for the sky or other bright area and live with the darker areas (possibly doing tricks in postprocessing), or add fill flash or the like. Or decide that there's too much contrast in the picture and you can't get a decent version of what your eyes are seeing.
This is why I say you want to watch your shot to shot exposure; you want to realize that the 1/100th at f/8 at ISO 200 exposure has to be wrong before you click the shutter. Even if you routinely check your histograms after taking a photograph, getting a good exposure to start with will save you the aggravation of immediately re-taking a shot (and trying to narrow in on the right exposure).
(Some people will say that the answer is clearly to use manual mode and maybe center-weighted or spot metering. Both are too much work for me and tend to blow up in my face in their own ways; in practice it's easier for me to watch shot to shot exposure and correct the metering with exposure compensation when necessary.)
Written on 19 January 2012.