Today’s Question: You said something in a recent post on exposure for RAW captures (November 16th) that goes contrary to what I have always been lead to believe. You said that you feel it is better to (if necessary) OVER expose an image without blowing out highlights to preserve detail. I was always thought that it is better to UNDER expose (if necessary) the image to preserve detail.
Tim’s Quick Answer: For a digital capture, it is indeed best to capture an exposure that is as bright as possible without blowing out highlight detail. That differs from exposure for slide film, for example, where it was generally better to err on the side of an under-exposure in part to preserve saturation and detail in the image.
More Detail: Part of the issue here relates to how the information contained within a photo is being captured. In a very basic way, you can think of more information as being “good”, at least up to a point. In other words, you want as much information (light) to be recorded as possible, without capturing so much light that you’ve blown out all of the detail in a photo.
In the context of slide film, a slightly dark exposure provides greater density, which in turn can provide greater color saturation and possibly more detail. Thus, the general rule of thumb with slide film is to slightly under-expose the image. You don’t want to go too far, but when in doubt it is generally better to expose slide film slightly too dark rather than slightly too bright.
With digital cameras, a dark exposure is generally not going to produce the best results. By capturing as much light as possible without actually clipping highlight detail, you’re capturing as much detail as possible. An image that is captured with an exposure that is too dark contains less information, and will exhibit more noise when you need to brighten up the image to produce a better final result.
So, with digital photography the best exposure in terms of maximum detail and minimum noise is an exposure that is as bright as possible without clipping highlight detail. This may, of course, require a bit of darkening of the image in post-processing, but in my mind that is a small price to pay in order to ensure the maximum possible detail and the minimum possible noise, all other things being equal.