Q: Most digital SLRs seem to have a dynamic range expansion function but I noticed that some also offer a High Dynamic Range mode and it's listed as a major feature. What exactly is this mode and how does it differ from the typical dynamic range expansion or optimizer feature?
A: The recent Pentax and Sony DSLRs (and the Canon Powershot G12) do provide an HDR mode, and it's very useful. When it's activated, the camera fire three shots - each with a different exposure level - and then merges them into a single photo. The final image exhibits dramatically increased shadow detail. This is quite different from dynamic range expansion, a function which provides greater shadow detail by extra processing of a single image.
For example, imagine a scene inside a cathedral on a sunny day. Small windows allow the sun to enter to illuminate some areas, while much of the interior remains significantly darker. A photo made with dynamic range expansion will exhibit lighter shadow areas than a conventional photo. Of course, the shadow lightening also makes digital noise (graininess or coloured speckles) more obvious. And when a very high expansion level is used, the photo can look unnatural. A camera with an HDR mode can provide a more dramatic improvement in shadow detail and as a bonus, image quality will be better.
Sony was a leader in providing a very versatile Dynamic Range Optimizer in its earlier DSLRs. The newer Alpha models (i.e., A55 and A33, both shown above) provide both DRO and an Auto HDR mode (that does not require a tripod) so you can use the feature that's most suitable given the lighting and the circumstances.
An HDR mode is particularly useful in extremely high contrast. Imagine a scene inside a cathedral or castle on a sunny day. Small windows allow the sun to enter, making some areas extremely bright. The unlit areas are significantly darker, of course. Shoot an HDR image and you'll get a great deal of detail in the shadow areas. The process should also tone down extremely bright highlight areas, but frankly, it's less impressive in this respect.
To illustrate the value of an HDR mode, I shot a conventional JPEG of this beautiful lobby and then activated the in-camera HDR mode. The HDR photo (a composite made up of three shots) definitely exhibits more detail in dark areas and is quite clean, without obvious digital noise. (Sony a55, hand-held; ISO 800; 1/15 sec.) (c) 2011 Peter K. Burian
Earlier DSLRs with an HDR mode required the user to mount the camera on a tripod when taking the three shots. The more recent Sony a55 and a33, as well as the Pentax K-5, allow for shooting the series hand-held. (The user can set the amount of exposure difference that the camera will use when taking each of the three shots.) Avoid serious camera shake - perhaps by bracing the camera against a solid object - and do not change the composition. As long as the three photos are quite similar in framing, the DSLR's processor can micro-align them so the final image should look perfect.
NOTE: Peter Burian teaches two excellent online photography workshops - Mastering the Digital Camera and Photography and Mastering the Canon EOS Digital Rebels - at BetterPhoto's online digital photography school. Also see Peter's Pro BetterPholio website: www.peterkburian.com.