Noise reduction. I have not tried this test myself, but I am going to.
The basic concept is to get the exposure correct in the camera when you take the photo. I know, sounds like a simple idea. Apparently, it can keep noise levels down. In the test the author purposely underexposed a photo by two stops at ISO 1600 and then adjusted the RAW file in software (Lightroom in this case). He also took the same shot at ISO6400. The camera used was a Canon 5D MkII. The image with ISO6400 in camera has less noise when you zoom in than the one adjusted in post.
Results may vary depending on the software, but Lightroom would be pretty much the same as Photoshop (same ACR modules).
http://photo.tutsplus.com/articles/p...voiding-noise/