> diameter of the aperture = focal length / aperture (f number) > so if we are considering an 85mm f3.5 lens we are talking of an aperture of > 24.3mm > so if we are considering an 85mm f1.4 lens we are talking of an aperture of > 61mm Karl That can't be right. Your formula for aperture does not take film format in to account and everyone knows it should. If you fit the lens to a digital camera the focal length increases so your calculations go awry. An 85mm lens is only 85mm on a 35mm camera (that is the modern definition ;o) Oh, and the real diameter of the aperture can be a *lot* smaller than the one you calculate (even if you ignore modern trends of mis-information). I guess the real thing we can't get round is that the hole at the front of the lens body (where the filters go) divided by the area of the image circle must come in to it somewhere. Ignoring other internal losses that mus set the upper limit to the speed of any lens You can of course make "fast" lenses cheaply if you can pursuade the consumer of the benefits of investing in a smaller "film" format ... which is exactly what "they" are trying to pursuade us to do right now ;o) Q
