On Wed, 2002-12-18 at 14:07, jcoyle wrote: > My understanding of the formal way to measure aperture is that it is the > ratio of the diameter of the _perceived_ aperture at the film plane to the > focal length of the lens in use*, and is thus at least as dependent upon the > effects of post-aperture elements on the bundle of rays which form the > image, as upon the diameter of the front element. This is not to say that > the diameter of the front element is irrelevant, just that you cannot expect > to measure it to support arguments that a lens has an effective maximum > aperture <4% smaller than it is rated.
Indeed, you are supposed to measure the size of the entrance pupil. This is the smallest diameter that you see as you look into the lens, or more formally, it is the diameter of the image of the limiting lens apperture when viewed from the front of the lens. By this I mean that all lens elements must be considered, and the one with the smallest image produces the entrance pupil. However, the entrance pupil cannot be measured directly, because it is inside the lens, and it is not a physical object. The entrance pupil cannot be larger than the front element, because if it were, the front element would be the limiting apperture stop and the entrance pupil. The size of the front element gives an upper limit to the size of the entrance pupil. Note the for normal and wide angle lenses, the entrance pupil is usually much smaller than the front element. -Scott
