Hi Pat,
I think I might have mentioned this before in another thread, but
do not trust what the 'white' (RGB) histogram tells you. It is an average of all three channels so is not a correct representation of the exposure for Ha. Press the 'info' button one more time and look at the red only channel in the split histogram.
The images below will show you why.... The RGB histogram in the first image looks underexposed, as its peak is just under the 20% marker and we should be shooting for around the 33% mark.
However, looking at the red only histogram in the second image... Oh dear, we are overexposed with the red channel peaking around 50%.
The highlights in this image were blown-out, but I was attempting to catch some of the fainter outer regions with this shot.
Apologies for the dodgy camera phone pics
