# "Aperture size is much smaller than focal length of the lens"

My theory is limited in this regard so I had a little doubt. I often see it mentioned in the problems that "assume aperture size of lens much smaller than its focal length" what does this signify? What would happens if they were comparable?

• it means that you assume that the paraxial approximation is valid but if the diameter were roughly the order of the focal length then you would have rays with large angle subtended from the axis Sep 21 '21 at 2:08