Arun H:
A 35 mm focal length f/2 and 135 mm focal length f/2 lens will both give you identical exposure with the same camera.
That's correct but the ANGLE that these optics collect is very different. The only reason they gave the same exposure time is that the 135mm has a quite bigger diameter and so photon collecting power. If You have a plane that emits light in equal manner (same number of photons/angle/time) and You image 1 degree angle of that, You will have a photon quantity but, if You image half a degree of it, You will have quarter of that incoming photons, no matter the F ratio.
