Exposure is at least as important in digital photography as it is in silver-based imaging, probably more so. As we've previously discussed, the dynamic range of today's digital, imaging sensors is limited. And when we lose highlight detail, the results are ugly. So, proper placement of highlight exposure is critical.
We know that specular highlights will throw off the exposure and cause us to underexpose the rest of the image. This is because those highlights are simply too bright. Not only is it futile to try to hold detail in specular highlights, those glimmers and shines aren't psychologically important anyway. We expect them to be bright. They provide punch to a photo and they're supposed to be blown out.
There are several ways that we can manage proper highlight exposure. Let's review them:
1. Take an Incident Light Reading: We must shield our exposure meters from specular highlights or compensate their readings in order to negate their influence on our camera's settings. The best way to do this is to forget about our camera's meter entirely and take an incident reading with a handheld meter. Incident readings will measure the light that is falling onto the subject instead of the light which is reflecting off of it. Handheld light meters aren't used as often as they once were. Yet, employing them is a foolproof way to deal with tricky lighting.
2. Take a Spot Reflected Reading: Incident readings do a terrific job of eliminating the influence of the surfaces in our image and their reflectance properties. But, they do not calibrate our exposure to specific tonalities as do spot readings. Please note that an averaging light meter is measuring the reflected light of the entire scene of our photo. And it is, at its essence, calibrated to yield a middle-gray tone in our imaging system. Middle gray! It's based on the premise that the scene is of average tonal values and distribution. Average out and blur the whole scene and it should result in a total image that is 18% gray—that's the entire premise of an averaged, reflected reading. But, if we narrow the point of focus (by using a spot meter or setting our camera for a spot reading) we can set any small spot in our scene to render middle, or 18% gray, in the final photo. (If you want that spot to be one shade lighter; then increase the exposure by one EV, etc.) If we do some quick arithmetic, we can also change the exposure to move that spot up or down on our grayscale. The Zone System, in part, relies on placement of gray values in this very way.
3. Bracket Your Photos: This is the solution that most people will use, particularly in the age of digital photography. It costs us nothing and we can set up our cameras to bracket shots any way we wish. It's relatively painless. When you encounter a scene where you have tricky lighting and/or lots of specular highlights, bracket your exposures. The brighter the specular highlights, the more drastically you'll want your bracketed shots to be.
While bracketing is the easiest method here, I fear that using it might make us more careless photographers. Yes, there are times when we must capture our images quickly and painlessly. But, by using solutions 1 or 2, it will make us better photographers. These are more thoughtful approaches that will aid us in our quest to previsualize our images. But, both incident and spot readings take time and care. They take an intrinsic understanding of light and exposure. They force us to slow down and to craft our images.
However, the bottom line is to get the shot. So, use whatever suits you and get the job done. This will save you time and effort and make your images better on many, many levels.