Hey guys, read what I wrote. I said f1.8 on crop is brighter than f2.8 on full frame when both have the same shutter speed and ISO, then you start telling me that I'm wrong to say FF is brighter when they're both at f1.8 and the same shutter speed and ISO. I didn't say that, so what gives? I went to great lengths to explain that at the same aperture, shutter speed and ISO, they both expose the same due to the different light gathering of the format being compensated for by the amplifiers being set different. If you can look past that same exposure settings between formats and start to use the higher ISO's with lower noise levels this lower amplification of FF rewards you with, you'll find the true nature of the light gathering of FF lenses on FF sensors.
80mm 1.0x [FF] @ f/2 aperture = 40mm diam pupil
50mm 1.6x [APSC] @ f/2 aperture = 25mm diam pupil
He is not.
Quote from: Wikipedia
In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.
You can't compare different focal lenghts. A 50mm is a 50mm both on crop and FF. However, being the former smaller, you crop the edges to a degree which would be equivalent to the angle of view of a 80mm on FF. This is a quick way to grasp the concept, but it's not exact in a way that you can make calculations.