Can anyone give me some tips on simulating a camera flash in After Effects. I am trying to make it keyframing the scale and opacity of a white shape layer but I can't get it to look right.
As a photographer, a flash is simply a very short burst of white light (as you're no doubt aware). The maximum time at full power is usually around 1/250th of a second, which is equal to 1/10th of a frame. In that case, if you were to accurately represent a flash being fired, you would simply boost the exposure of a single frame by several stops (to make it almost completely white, whilst retaining a little bit of detail).
Something as simple as this would suffice.
For one frame.
If you really wanted to be technically accurate, you could simulate the CMOS flicker of a flash by only boosting the exposure on a horizontal portion of the frame, say the top 2/3rds or something.