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Monday, June 18, 2007

Underexposure & Digital Images

Myth: Images must be underexposed to prevent highlights from being blown out, and if one is shooting RAW, this underexposure is easily corrected later

Underexposure & Digital ImagesThis myth can cause some serious color and noise problems for the photographer who wants to get the highest-quality images from a digital camera.

On the surface, keeping highlights from being blown out is a good idea. Once their brightness passes the threshold of a sensor, detail is lost. No amount of Photoshop work will bring back the detail in those highlights, though there are some fixes that can fill in washed-out highlights. For an efficient workflow, you never want to needlessly increase your work in Photoshop.

Recently, I'm seeing an overcompensation of this idea, however. Photos are underexposed to the point that no overexposure flashing warnings appear in the LCD on image review, and the histogram is nowhere near the far right edge. (Note: Flashing warnings are not precise measures of overexposure.)

Let's look at what happens when such underexposure occurs. Consider that a sensor is designed to respond to a certain tonal range from black to white in an image. If the full range of the sensor isn't being used when the image is captured (underexposure), then some of the capabilities of the camera haven't been used—capabilities that you paid for!

Now, that might not seem so bad. After all, especially when shooting in the RAW format, there's often a lot of excess data anyway, which can be used during image processing. Underexposure actually is a much more serious issue, however.

A sensor does its best job of capturing bright colors if they're exposed to keep that brightness registering fully on the sensor. As colors are underexposed on the sensor, they obviously become darker. If you change exposure on a color, it appears darker or lighter in the image, but clearly, the color doesn't change in real life. More exposure reveals the original color.

This isn't true with the underexposed image itself. Increasing “exposure” in the processing, such as using Camera Raw, makes the color brighter, but doesn't reveal the original color. As a color is underexposed (and gets darker), the sensor doesn't respond the same to the chroma or actual color information in the image. There's less chroma with the underexposed color, offering less color to work with when processed in Photoshop.

Underexposing also forces tonalities into a smaller range, which especially affects darker tones and colors. When processed, these tones can be expanded, but contrast will be increased and subtle tonalities can be lost. A lot of photographers say “so what?” to this because of RAW, but I'll guarantee if you shoot a test that includes subtle colors and tones, you'll see them change from proper exposure to processed underexposure.

This can be a problem particularly with higher-pixel-count digital SLRs. These cameras are squeezing more pixels into the sensor, meaning the individual photosites must be smaller. Physics alone means fewer photons of light energy can be seen by each photosite, which can restrict the sensor's ability to handle colors. Camera designers have done incredible things with these sensors that people said would be impossible a few years ago, but they're limited by the physics of light. So if you underexpose these cameras, you're only stressing the sensor to try to deal with colors at less than optimum brightness.

Another image artifact affected by underexposure is noise. Today's digital cameras do a remarkable job of controlling noise and allow higher ISO settings with less noise across the board. The problem, however, is that noise is best controlled at proper exposures. Under-exposure always will increase noise, at least to a degree, and as underexposure increases, noise can increase dramatically. I've seen the same camera deliver a nearly noise-free image when exposed properly, yet when underexposed, the noise increases so much that you'd swear the ISO setting was changed.



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