Monday, May 10, 2010

Ten Reasons To Overexpose—05/31/10

DPMag Published in Tip Of The Week
Ten Reasons To Overexpose—05/31/10

This Article Features Photo Zoom

I recently delved deep into a topic I’ve long heard photographers discuss but had never seriously tested for myself: overexposing RAW files. It’s something some photographers have strong opinions about, and rumors and hearsay abound. I thought finding out the facts might be the most appropriate way to learn for myself, once and for all, if it’s really better to overexpose digital images. Now I know for sure, and I’m sharing 10 tips that can help you overexpose appropriately to maximize detail and minimize noise.

1. Overexposing can in fact be the ideal approach, but only if you’re capturing RAW files. After capture, adjusting the image to make it look “normal” in the RAW processor is a crucial step, and to reap the benefits of overexposure it has to be carried out in RAW rather than JPEG shooting mode.

2. Overexposing RAW images works because it increases signal-to-noise ratio. More image-forming detail is contained in the brightest areas of the image than in dark areas. If you think of a pixel as a bucket, and light as the water filling that bucket, the more water in the bucket, the more information you have. All that “water” is the equivalent of a stronger signal, thereby improving s/n ratio. And you always can eliminate excess water from the bucket later, which is the principle at work when you overexpose RAW.

3. Overexposing only works as long as you don’t go too far. Continuing with the bucket analogy, it doesn’t work if there’s overflow—and that’s what happens when you overexpose too much. When you overexpose bright highlight areas of the image, they can clip and lose all detail. Once detail is gone you’ll never get it back. So the key is to overexpose enough to brighten the image but without clipping highlight detail.

4. Overexposing appropriately requires checking the histogram on the back of the camera. The process is sometimes called “exposing to the right” or ETTR, because it involves moving the peaks on a luminance histogram toward the right side of the graph (representing the brighter portion of the image file). If the peaks move all the way to the right edge and go beyond it, that’s when detail has been lost due to clipping. So move the peaks toward the right side of the histogram without going too far.

5. Overexposures should be brought down to “normal” in postprocessing of the RAW file without increasing noise. In fact, noise is minimized (especially in the shadow areas of the image) with subtle overexposure. It actually produces a cleaner, more noiseless signal than the “correct” exposure indicated by the light meter. That is, as long as you don’t overexpose so much as to clip the highlights!

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