Roving Thoughts archives

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.)

Link: A demonstration of an issue with ETTR

This link requires a bit of explanation (if only so that I can remember it later). The person I'm linking to took a standard colour checker test target and took a properly exposed shot then a series at increasing positive exposure compensations (ie, exposing to the right), postprocessed all of the overexposed photos to correctly expose them again, and then cropped strips of the colour targets and stacked the same strips from each exposure. The goal is to clearly see any colour shifts caused by ETTR.

(As he notes, some are deliberately overexposed, going beyond what you should theoretically do with ETTR. Of course they do represent what happens if you accidentally overexpose in the course of trying to do ETTR.)

Part of what I like about this is that it's an experiment that anyone can do (if you have a colour checker test target). Your camera and processing system may well give you different results than his, but either way you'll have learned something interesting.

So: Experiment: ETTR hue shifts and now the revised Experiment: ETTR hue shifts (reformatted).

PS: obviously you should do this test with the camera on a tripod and locked at a single ISO, unless you also want to test the effects of ISO on colour shifts. Although that too may be a useful test, depending on how you're thinking of using ETTR.