Oddness with AV

Yes, I was immediately struck by your 0.73 figure.
The power is very precisely 0.5.

It would be interesting to pursue this as I may have time to do my own tests tomorrow.

What programme are you using to analyse the images ?

How do you move this discussion to a new  thread ?

What programme are you using to analyse the images ?

I was using HDRShop which was convenient for grabbing the average and sum pixel values.  It also allows you to change the response curve when you open images, which I had been setting to 1.
Gimp should work just as well.  I started looking at the histograms in gimp, plus median values and the results seem the same.

How do you move this discussion to a new  thread ?

This I don't know short of starting a new one.