Some days ago, I decided to take some pictures of the Moon, was my first time. I used my 100-400L with a 2x extender (see tags and exifs) for a 800mm focal length on my 5D Mark II. Both pictures have been cropped, so you visualize here almost a 1500mm focal length. During the session, the Moon was widely masked by thin and low level clouds, the light then was hugely filtering into and a vast halo was visible around the Moon. It was clearly not the best shot conditions (neither the best location).
Even if this halo was not unpleasant, I decided for this first try to remove it by adjusting my shot. This picture was 95% adjusted using levels, the only other tool used was the one to recover details on highlight from Aperture.

On the following picture (taken two days after), I've exposed on the "right" and get the details back during post-adjustments.