On Jun 28, 2006, at 7:23 PM, John Francis wrote: > Well, that was kind of the point. F/4 is always supposed to be f/4. > But if one lens had more losses in the optical path (poor coatings, > internal reflections, etc.) it might end up only letting through as > much light as a better lens at f/4.5 > > Personally I very much doubt if any Pentax lens loses anywhere > near even 1/3 of a stop in the centre of the image area, let > alone there being a noticeable difference between a zoom and > a single focal length lens. Where the zoom lens does often > perform poorly (especially a fast zoom such as a f/2.8) is in > the corners of the frame. Even the acclaimed FA* 80-200/f2.8 > shows significant light drop off in the corners when used at > f/2.8 (as do the comparable zooms from other manufacturers).
The f-stop is a calculated value, not a measured one. It is simply a ratio of the apparent diaphragm opening to the focal length. So f/4 will always be calculated the same way, but not all f/4 lenses transmit the same amount of light. For this reason, lenses for the motion picture industry are usually marked in t-stops (transmission stops), a measured value. A few lenses for still photography have been marked in t-stops. In an ideal lens, f/4 and t/4 would be identical. In practice f/4 always passes less light than the theoretically ideal f/4 lens. Zoom lenses, because they have so many elements, usually show a greater difference between marked f- stop and measured t-stop. Maybe that's what the fellow on eBay was talking about, but more likely he just doesn't know what he is talking about. Bob -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
