Today’s Question: How do I know when I should be capturing several images for an HDR (high dynamic range) instead of just a single exposure?
Tim’s Quick Answer: Conceptually, whenever the histogram display for a single exposure shows clipping at both the black and white ends of the histogram (or significant clipping at one of the two ends), you might want to capture bracketed exposures to possibly assemble into a high dynamic range (HDR) image.
More Detail: With practice, of course, you can learn to anticipate when the overall contrast (or dynamic range) of the scene you are photographing is significant enough that a single exposure won’t be able to contain the full range of tonal values in the scene. The limitations in terms of dynamic range vary from one camera to the next, but over time you will develop a sense of when you’re near (or past) the limitations of your camera.
Of course, just because the dynamic range of the scene is beyond the capabilities of your camera to record in a single capture doesn’t mean you should create an HDR image. A photo with clipping of detail in the darkest shadows could simply be a dramatic photo, and a photo with clipping of highlights could simply be a very nice “high key” photo.
That said, my approach is to capture a sequence of bracketed exposures whenever I know (or my camera’s histogram display tells me) that the full dynamic range of the scene can’t be captured with a single photograph. This provides the flexibility of deciding later whether I want to create an HDR image or simply work with the best of my various exposures.
So, there is an element of experience in terms of being able to anticipate when the dynamic range of a scene exceeds the capabilities of your camera. There is also an artistic decision to be made here in terms of how you will interpret the scene. But in general I prefer to have bracketed exposures whenever the scene has a high dynamic range, so I have more flexibility when processing my photos later.