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Underexposed - 24MB file
RAW image capture gives photographers the versatility to make subtle image adjustments in post-production as effectively as if they’d been made in the camera before the capture was made. The benefit there being that over or under, you can make an exposure look perfect long after it was made. But the principle of overexposing RAW actually goes farther. It’s based on the idea that in a brighter pixel there is more image-forming information than in a darker pixel. Because of that, if you make all of your pixels a little bit brighter during capture—so long as you don’t overexpose and eliminate detail in those brightest pixels—you can bring them down in post while retaining all that extra image-forming detail. It’s a great way to maximize detail and minimize noise—especially in the shadows.
Brighter - 25.8MB file
Don't believe me? Take a look at these pictures of my beautiful baby girl. The underexposed image clocks in at 24 megabytes in the original RAW file, while the brighter image is 25.8 MB. That may not sound like a lot, but it's 7.5% bigger. That means 7.5% more information contained in the slightly over image than the slightly under one. And that translates directly to less noise and more detail.
Some folks call this technique "exposing to the right" because it's about moving the peak on a histogram toward the right—or brighter part—of the frame. If your histogram peaks are on the left, your image is dark. If your histogram peaks are on the right, your image is bright. As long as your histogram peaks aren't clipped against the right edge of the frame, you're not throwing away detail in the highlights through overexposure. Don't do that, because there's no way to get those details back.
But if you just move the peaks on that histogram to the right through subtle overexposure, then use your favorite RAW processing software to move the exposure back to normal, you'll find lower noise and higher detail in all your photos. If you still prefer to shoot for a perfect exposure every time, think of it this way: if you're going to err, err on the side of overexposure. Err to the right.