This shows how to understand the principle of equivalence by doing a kind of reverse engineering.
Two lenses are used a 90mm and a 135mm both are set to f/4.
The idea is to fix the field of view, the picture one would like to take. I am doing it using an iPad here.
Both images are projected on a white screen . This show that for the same field of view one would need a smaller detector with the 90mm lens, a x1.5 smaller detector which correspond to the ratio between the two focal lenses.
Also, since the f-number is the same, one can check that for both images the light density is almost the same. Therefore one can understand that the picture made with the big detector is made out of more light.