Hamlet's new D-SLR dilemma: to underexpose or not to underexpose?
By Rob Sheppard
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).