Sarah Guarino, 9to5 Mac:

To do so open up the Camera app. Then you are going to tap on the focus point. The focus point is the subject point of the picture. When tapping to focus, a yellow square appears around the subject. To adjust the exposure level, tap on the yellow sun and slide it up and down. Sliding the sun up will make the picture lighter, and sliding the sun down will make the picture darker.

Excellent tip for a very cool and useful feature. It must have been mentioned in some of the iOS 8 reviews, but somehow I missed it.