Today’s Question: I wonder if you could clarify the benefits of exposure bracketing. Is the benefit simply to have more options? If I take three bracketed shots and then merge those photos in Photoshop, will the result simply be the same as the middle exposure? Or is there a gain to taking and then merging bracketed shots?
Tim’s Quick Answer: There are two key reasons to bracket exposures. The first is to have an additional option in case the base exposure isn’t optimal. The second is to be able to blend exposures for a high dynamic range (HDR) result.
More Detail: Bracketing exposures simply means that rather than capturing just a single exposure, you will capture additional exposures that are either brighter or darker (or both) compared to the baseline exposure. A standard approach, for example, would be to capture both an exposure that is one stop darker and an exposure that is one stop brighter, along with the standard exposure.
One of the primary reasons photographers might bracket exposures is if they are dealing with a tricky lighting situation (such as backlighting) and they aren’t confident that they will get the exposure right. In some situations you may obviously have the time to capture a test exposure and then capture additional photos with adjustments as needed. But in many cases you may not have that flexibility. For example, with concert photography things may be changing so rapidly that if you don’t get the shot right the first time, you won’t get the shot at all.
So, bracketing exposures helps give you options so that if the baseline exposure is problematic, you have other exposures that will likely have worked out better. Note, by the way, that while bracketing with a total of three exposures is pretty standard, many cameras will enable you to automatically bracket with five, seven, or even more exposures.
The other common reason to bracket exposures is for scenes where the overall tonal range exceeds what can be captured in a single exposure. For example, when including the sun in the frame it is often impossible to capture a single exposure without clipping the highlights or the shadows (or both).
When bracketing for HDR I generally recommend separating the exposures by two stops. In many cases three exposures may be enough, but in extreme cases you may need to bracket with more exposures. The bracketed exposures can then be assembled after the capture to create a final HDR image that contains more detail than a single exposure would have been capable of. Note, by the way, that if the tonal range of the scene doesn’t exceed what your camera can capture with a single frame, there won’t be a benefit to creating an HDR image. In fact, in this situation creating an HDR image may result in more noise than would be present in a single exposure.
