Today’s Question: One thing I’ve never understood is all the stacking for HDR. I’ve done that and it’s cool and all but in normal conditions I simple take two exposures one for the darks and one for the light and then blend them. What is the purpose of taking more than two shots when you can capture all the darks and lights with two?
Tim’s Quick Answer: For high dynamic range (HDR) images you only need enough exposures to cover the full range of light from the darkest shadows to the brightest highlights. If two exposures are enough, there’s no need to capture more.
More Detail: You can most certainly assemble two exposures into an HDR image, or even just blend the two images with a layer mask in Photoshop. However, more often than not I find that in situations where a single exposure won’t cover the full range of contrast in a scene, two exposures isn’t quite enough either.
To start with, I recommend separating exposures for HDR by two stops. If you separate by more than two stops, you may not have enough overlap in the tonal range for the exposures, causing problems for the final HDR image. You can separate exposures by less than two stops, but there’s no benefit to doing so.
Another consideration is that it is often easiest to use automatic exposure bracketing (AEB) to capture the exposures for an HDR image, in part to enable you to have those exposures captured in a short a time period as possible to avoid changes between exposures in the scene you’re photographing. Most cameras default to three exposures for bracketing, though many cameras offer additional options such as to bracket five, seven, or nine exposures.
I often capture bracketed exposures in sets of five images, just to provide some insurance with the bracketing, and to avoid needing to frequently change the camera settings to capture a different number of bracketed exposures. But if you end up with more bracketed exposures than you really need, you can exclude the unnecessary exposures when assembling an HDR image or even delete the exposures you don’t need.
