Regarding flat frame taken using AV mode at low iso.
I would think that the 'aim' of flats would be to have the frame fully, and evenly illuminated, rather than having dark corners, as per my image. Surely that will defeat the purpose of taking them, yes?.
Am I correct in thinking that?
That's a perfect flat as far as I can see. The purpose of the flat is to remove any vignetting that exists in the optical train. This will correct your lights perfectly.