I know you've heard this or something like it: "I always underexpose my digital files. I want to be sure my highlights are protected, so underexposure, for me, is like insurance for the highlights. Anyway, I shoot RAW, so it really doesn't matter if my image is underexposed."
Let me tell you straight out that I think the cost of this insurance is too high. Sure, it isn't an awful idea. It's possible to get a good photo from underexposure. It can take a little more work, but you can do it. And some pros who use this technique get excellent images.
But there are problems that come from a blanket use of underexposure. There's an old saying among sales people that problems are simply opportunities. In this case, though, I don't think there are many opportunities coming from unnecessary noise, higher contrast, color changes and more work needed in the computer.
I don't like these problems, so I avoid underexposure whenever I can. That doesn't mean I overexpose my highlights, either. It means I expose carefully to get the most from my sensor's capabilities. Think about it. You spent a good amount of money for your digital camera, and for most higher-end cameras, a lot of that cash went toward the sensor and its associated circuits. Why wouldn't you want to get the most from that investment?
There are a few pros who want to take their old slide-shooting ways and apply them to digital, so they recommend underexposure as a safety measure. But I can tell you from looking at some of their actual files that these pros often are losing color and tonality in the low end because of unneeded underexposure, in addition to increasing noise and contrast. Let's quickly go over these problems in more detail (all apply to both RAW and JPEG image capture).
Noise. Today's digital cameras do a great job minimizing noise—as long as the files aren't underexposed and then significantly lifted in brightness in the computer. If areas dark from exposure really need to be much brighter, you'll see noise increasing in all digital camera files. This is especially
a problem with higher ISO settings.
Higher Contrast. When an image is underexposed, fewer steps of tonal value are recorded (because the darkest tones become black). That gives less tones for Photoshop to work with when the bright areas are lightened to their proper tones (you can get only one tone from black—black, no matter how many tones were buried in that underexposure).
Less Range To Work With. Just like the previous step, underexposure results in fewer steps of tonal range. This amazes me when people say, "But I'm shooting RAW." I have to think, "And you're not shooting JPEG, which has fewer steps of tonal range, because you can underexpose RAW to get fewer steps of tonal range, and which will require more work in processing.
Color Problems. When rich midtone colors are exposed as dark colors, they often lose saturation and chroma (their color quality). When you move them back to their proper tones in the digital darkroom, they rarely have the proper colors, so they need more work.
Lost Colors And Tones. As noted above, when dark tones and colors are underexposed, they can get to black, meaning they're lost forever as far as that digital file is concerned.
I believe in exposing for the needs of an individual shot and being sure I have the detail where I need it. Some still say, "But I shoot RAW, so I can do this." I have to be honest, I cringe at the idea that RAW cures all and that it should be underexposed.
RAW is limited by the sensor's capabilities just as JPEG is. You can often expose brighter with it than JPEG because you have more steps of tonality and color in the bright areas. (So again, why would one shoot RAW versus JPEG for the added tonality, yet underexpose to deliberately restrict it?) RAW gives you more flexibility in processing and in getting more out of bright and dark areas, as well more capabilities in doing strong adjustments without damaging the image quality.
Underexposing does make more work for you in your RAW converter and in Photoshop, plus you're often shifting colors. I got into this whole business of digital because I thought it offered great possibilities for photographers, not because it let me sit in front of a computer. I have no desire to spend additional time working files because I didn't expose them right in the first place.
People will talk about how they underexpose, use RAW, then choose Adobe RGB (1998) for its bigger color space compared to sRGB. I can tell you that I can probably get better colors with less work going to sRGB if I've exposed correctly than underexposure with Adobe RGB (with more work, I might come close). If you doubt that, just try it.
I never arbitrarily underexpose scenes unless I find through testing that a camera and its meter are consistently giving me too much exposure—and that's no different than when I shot film. And I admit that I've made mistakes in this area. I once screwed up part of a pro shoot many years ago because I didn't check how a new camera metered (but that's another story). And I'll guarantee that you won't find problems with my highlights and bright textures simply because I don't overexpose them. Not overexposing important bright areas isn't the same as underexposure.
For me, underexposure without a deliberate reason for a specific camera or image is poor photographic craft. I say this not from my own experience as a photographer, but from working with so many images and photographers who we publish in our magazines. With our magazines, we did a lot of trial and error with early digital image files and learned a lot about them. I also know from talking to our art directors and my book publishers that they consistently like working with my image files because they go to print so well.
I've heard from participants in my classes that they have been told there's a rule that one doesn't overexpose highlights. Highlights are sacred and must be preserved! So maybe underexposure for insurance isn't so bad. We must preserve the sacred.
Okay, so I don't much believe in "rules—I sometimes suspect that comes from being a teenager in the '60s! Guidelines are helpful; rules, to me, can be problems in creative fields where there are so many variables. Trying to fit photography into arbitrary rules makes it way too automatic for me. It's like stereotyping people because "most" of a certain "type" act the "same."I believe in the power of the photograph. I care about what a photograph does and how it impacts a viewer, not how it meets some arbitrary standards. I can guarantee that no one is going to look at your photo and wonder why highlights are blown out if it looks good.
What makes your photograph, your subject, your composition look good? Only you can know that. It's true that it's worth learning why blown-out highlights (or any other problem) can give you trouble and how to expose to keep them looking good when needed. If bright highlights wash out your image and make it look bad, distract from the subject and keep your viewer from seeing your image properly, then they're wrong—but underexposure might not be the answer. A different angle to the subject, a different light might be better.
On the other hand, if you expose to keep unimportant highlights, making your image dark so that it looks bad, distracts from the subject and keeps your viewer from seeing your image properly, then that's wrong, too. Every technique must be mentally processed by the photographer to be sure it fits a particular subject, composition and purpose for the image.
Bottom line: Sure, you can underexpose for "insurance" and still get good images. Pros who do this prove that's possible. However, that doesn't mean you're getting the best images or using the investment in your camera to its best advantage. To me, it's like keeping your speed at 55 mph just to ensure you don't go over 65 mph because you might get a ticket. Why not just go at 65 mph?
You get good exposure by watching your histogram to be sure there isn't a big gap at the right side, by letting bright and unimportant highlights go bright, by using the exposure warnings smartly (not always trying to get rid of them, but trying to minimize them or at most, changing exposure until they just disappear) and so forth. You can also get exposure insurance by that good old film technique: bracketing your exposure. Most digital SLRs include it as a menu option. This can guarantee you get the right exposure.
You get better with experience—exposure is a craft. Ultimately, exposure that gives you the proper color and tonalities for your scene, as best as you can manage it, is the best exposure