>f-number has absolutely nothing to do with beams of >light, axial or otherwise. Nor does it have anything >to do with light transmission, vignetting or filters. The included ange of the cone of rays converging at a point on the focal plane determines the depth of field. This in turn is determined by the numerical aperture (as well as the focal length and subject distance). >An f-stop is simply the ratio between the focal length >of the lens and the diameter of the aperture. Nothing >more, nothing less. But life is not that simple. See below. >It's not at all unusual for a zoom lens to transmit >less light than an equivilent fixed lens at the same >aperture. All those extra glass surfaces increase >internal reflection and reduce transmission. With >modern lens coatings the effect is much smaller than >it used to be, but it's still there. I have always assumed that the most important aspect of lens aperture was the following desideratum: "When a lens is set to a given aperture or reports that aperture to the camera, it shall transmit the same amount of light as a _theoretical lossless_ lens at that (geometrically defined) aperture." An honest lens designer/marketer would design his lens to behave this way. It implies that he would design his (real-world) lens with enough extra _geometrical_ aperture to compensate for its internal transmission losses. Thus, if he designed a lens whose maximum aperture would be advertised (both to the consumer and to the camera exposure electronics) as an f/2.8, the geometry of the lens might have to be f/2.4 to _transmit as much light_ as a theoretical f/2.8. Sure. This means that the lens actually exhibits less depth of field than the theoretical lossless f/2.8 lens, but it does something that is more important: _it transmits as much light_. An unavoidable tradeoff, and that's the direction I would want it in. If lens designers went around calling their lenses by the geometrically defined aperture, and those lenses reported the same to their camera bodies, how could the designers of the exposure electronics know how much transmission loss to factor in? The information on a given lens would be unavailable; it would have to be assumed as some kind of brand or industry average. Is that what they really do? The only way out, if they really do all trade on geometrical aperture, would be if the transmission loss were negligible; I believe, as you do at least for zoom lenses, that it is not. DGW * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
