Why Is an f/1.2 Lens so Dark on Digital?
How can an f/1.2 lens be as dark as T/1.5? This is no simple vignetting behavior, but notably more involved, namely the properties of a digital sensor with respect to ray angle.
I’ve added a new page Loss of Lens Brightness at Fast Apertures on a Digital Sensor to Making Sharp Images. The page shows three scenes using two lenses on two different digital cameras.