We handled focus yesterday (see Focus Subtleties article). Today, let's tackle exposure the same way.
As I've written before, there are no "magic settings." Exposue is probably the second most "unmagic" of them all, as leaning on the automatic abilities of the camera will cause you either major or minor problems, depending upon the situation.
That said, there are some common things that get lots of people in or out of trouble. Let's look at a few of them:
- If the light doesn't change, the exposure shouldn't either. My friend Chas Glatzer (www.shootthelight.com) teaches this right up front in his workshops, and you should listen. If you're photographing in a situation where the light isn't changing, then your exposure shouldn't change. Yet virtually all of you are using matrix metering, which evaluates subjects and changes exposure based upon unpublished and highly complex evaluations. In some cases--the D80 is well known for this--matrix metering actually overcompensates for subjects (what's under the focus sensor being used). But in all cases, two successive frames with matrix metering may very well be exposed different in unvarying light. Warning Will Robinson, Warning!
- Subjects do move in and out of light, though. If your subject is moving from shade to sun or vice versa, then perhaps the exposure should change. There are reasons to use an automated exposure method like matrix metering, but make sure that the sitaution you're photographing is one of those where the reason applies.
- Old methods still work. Most of us who used slide film used spot metering on a midtone object to establish exposure. This still works, though the definition of "midtone" is a little more complicated than it used to be because gamma is now involved in how your image looks (that's the Brightness setting in Picture Controls for you JPEG users). But, assuming you know what you're doing and what your camera will do with what you set, spot metering midtones still works. On digital, I'd say you should also spot meter highlights to verify that you're keeping them where you want them, too, but that's starting to get a little too far down the expert path for this short article.
- Don't 100% trust your histogram. Unless you're using UniWB, the histogram the camera presents is showing channel values that have been swung via blue and red channel compensations, not the underlying raw data itself. Thus, the histogram may tell you that you've blown out highlight data when you haven't, or that you're preserved highlight data when it isn't. The more you deviate from Neutral, Contrast -1, and a white balance of somewhere near 5000K (actual value varies with sensor), the more likely that the histogram is off enough to mislead you (and it can mislead you with those settings, too). On the flip side, the histogram is one of the few tools you have to evaluate your exposure decisions, so spend the time to learn what it is that it really tells you. It's a useful function, just not 100% reliable. Understand that difference.
- White balance is important for exposure. Note what I wrote above: the red and blue channel data is changed to adjust the white balance. So if you meter with the wrong white balance, you're definitely getting skewed data in the histogram. It may not seem overly important, but just as setting white balance comes before exposure in the workflow for raw file conversion, so should it be for taking the picture in the first place. Thus, for JPEG users: set white balance first, then exposure, not vice versa.
- Flash requires two exposure calculations. And both have to be "right." Nikon's bodies do a terrible job of telling you what's going on when using flash, and the restrictions imposed on shutter speed and aperture get in the way, too. The metering bar is only showing the ambient (background) exposure, for instance. The flash will light only one distance (subject) correctly, and generally that distance will be the focus distance. Just make sure you remember to evaluate both those things separately.
- Know the locks. Most consumer compact cameras tend to "lock" exposure when you partly press the shutter release. Nikon DSLRs do not do this unless you've used one of the Custom Settings to change this behavior. Thus, if you're moving between a compact and your DSLR all the time, it behooves you to know when and how exposure locks are being applied on all your equipment. It's likely to be different! Likewise, there are some subtle differences in how the AE-L button works on Nikon DSLRs. The button can be programmed to do several different variants of exposure locking (e.g. only during press, starting with press and lasting until shutter release, starting with press and staying until cancelled). Moreover, there is a timeout involved in the AE-L button for some settings. Make sure you know which function you've assigned to this button before relying upon it.
- Vignetting is not really exposure. One common complaint I get is that two lenses of the same focal length and same aperture give "different exposures." Any number of possibilities can produce this (keep reading), but if you're basing things off a histogram, vignetting sometimes can shift the histogram values enough so that it looks like an exposure change. I've seen as much as a one-and-a-half stop difference in the corners between some otherwise equivalent lenses, and that difference is going to show up in the histogram. But that doesn't necessarily mean that the exposure should be different for the two lenses. This is another area where matrix metering can get tricked a bit (and the reason why center-weighted metering existed in the first place instead of frame-wide metering).
- f/stops are not t/stops. If you rely upon on a handheld meter to set exposures, this one will catch you. The 200-400mm f/4 is marked f/4. But in terms of the light it transmits all the way to the sensor wide open, it's more like 5, and we label that t/5 (the t is for transmission). This doesn't really come into play for in-camera meters, because they're metering through the lens, so any light loss is already present before it gets to the meter. But, again, if you use handheld meters, you need to know the t/stop value of your lens, not the f/stop.