17 Aug 2013

Automatic Exposure Bracketing (AEB) and HDR

Submitted by Roger

Your eye has an amazing sensitivity and range. This is because it is always moving, always sampling the scene. Your brain is equipped to blend all the little pieces of information into a composite image that encompasses the full range of detail from extreme highlight to extreme shadow.

However when you photograph that dramatic sunset, you find that the foreground becomes an uninteresting black silhouette. Your eye can see the details in the shadows, but the camera cannot capture them while correctly exposing the sky.

The camera can only work with a single exposure. To achieve a higher dynamic range in our photography, more closely approximating what we see, we need to mimic the role of the brain by sampling multiple exposures and combining them in post-processing.

The following photographs were taken using a tripod to keep the camera steady, and using the internal camera AEB (Automatic Exposure Bracketing) option.

The camera was set to Av mode (Aperture Priority) to allow the camera to vary the exposure using only the shutter speed. This eliminates any depth of focus artifacts which might arise. The exposures were bracketed by two stops.

The first shot maintains the dramatic sky, the second the midtone structure in the trees, and the third allows the foreground detail to come out. In the first shot, the trees are blending into the sky, and the overexposed sky in the last shot is spilling light around the branches, obscuring the tree structure.

Combining these three shots in Photoshop Elements is simple.

First, load the three shots into the editor. Select the three images in the Project Bin by holding the Ctrl key and clicking on each. Then go to the menu and select Edit>New>Photomerge Exposure.

In the Photomerge Exposure screen, choose Automatic and select Smart Blending. We set the Shadow Detail slider at 100 to bring out the grass in the foreground, and the highlight detail slider was at 78 to get the best detail in the sky. We boosted the saturation slightly to bring out the delicate pinks and oranges in the sky, and clicked on Done.

digital photography
exposure bracketing