You’ve probably read elsewhere about why you should expose to the right. If you haven’t come across this before, the basic idea is that you “overexpose” as much as possible without clipping any of the highlights. On most modern, certainly DSLR, cameras, your rear LCD will indicate if you have any overexposed pixels. Once you get the image into your favourite image editing tool, you then set the image back to its correct exposure. By the way, this only works if you are capturing in RAW.
The theory behind this about the way digital sensor capture “photons” of light and turn them into pixels. The number of levels of intensity available to the sensor is less in the darker area of the scene than in the brighter. So if most of your image is exposed to be bright, then there are more “graduations” available to you and therefore less noise. Ok, so this isn’t quite what’s going on but good enough to get the idea. Much more (correctly) technical explanations are available via a quick search on Google. Or alternatively, you could try this article on Luminous Landscape which is where I first came across the concept. I’d recommend going over there anyway, it’s a great site.
To illustrate the quality difference, the following two images are crops from a DLSR “scan” of a scene from the London Eye. The original was 35mm Ilford HP4 (I think). The first is as my 5d mk ii metered the negative. In the 2nd, the image was “overexposed” by 2 stops in the camera and then set to -2 stops in Lightroom. If you aren’t sure about DSLR scans, I blogged about it some time ago.
As you see the noise in the second image is much less than in the first. The highlights show a similar difference, although not quite as pronounced. It’s not always possible, to expose the right, but if you have time to take a few shots to try and achieve maximum quality, it’s well worth trying it out.