iPhone 4 camera - how does the HDR mode work? I recently got an iPhone 4. When I turn on HDR on my camera and take a picture, it takes 2 pictures, both of which look grainy.

One appears brighter than the other but I don't "see" any other difference.

Any idea why this happens or is this how it is supposed to be?

Thanks.
Here is an article that provides a really good run down on what HDR is and how it performs on the iPhone.

Basically it takes three photos at various exposure levels and combines them.  So you get a "normal" photo at regular exposure levels and the HDR version (1 photo composed of the three).   Results vary depending on light conditions etc.  I've had some really great results and some shockers.