The f.no. calculation assumes that the lens is a simple design, eg. a single element or more likely a symmetrical design such as view cameras use. In practice a lot of long lenses are of the telephoto design which means they have a teleconverter built in.
I bet the front element of this lens is less that 200mm from the film plane. Imagine if you drew a cone with its apex at the centre of the film plane and passing outwards through the edges of your lens. When this cone intersects a plane that is 200mm in front of the film plane it will make an imaginary circle that is the theoretical aperture of the lens. Beware of the conspiracy theorists and other alarmists on this mailing list. If a reputable manufacturer like Pentax says that a lens is 200mm then its sure to be within a few percent of that number. It is unlikely to be as under-length as 180mm, after all that's a common enough focal length from other manufacturers, so it's hardly credible that Pentax would fudge its naming to hide the lens's true statistic. Regards, Anthony Farr ----- Original Message ----- From: "Frank Wajer" <[EMAIL PROTECTED]> > Hi, > > on kmp.bdimitrov.de I saw the specs of the 200 f2.5 lens. It says that the > filter > ring is 77mm, but this means that the lens cannot be f2.5, as that would > mean > a minimum 80mm filter ring! > > Can anyone explain? > > > BTW. Isn't it save to have some margin between the filter ring and what is > calculated by dividing the focal length and the aperture. I'm thinking > about > robustness against vignetting, especially when using a filter. > > Frank > - - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
