Joseph Tainter <[EMAIL PROTECTED]> wrote: > (...) > So, are the SQF/MTF tables done at some distance other than closest > focus? Which distance?
Hi Joe, The magazines I know usually test the lenses focusing them at 20 times the focal lenght (300x20=6m). This is probably something Mike Johnston can say with more knowledge (when I reviewed lenses for the magazine I worked for years ago I never had the chance to do a MTF test, only real life shooting). > How do I calculate where, on the distance scale, the sharpest aperture > changes from f5.6 to f16 at 300 mm? That's an interesting question. It should depend on the optical construction of the lens. My father's 28-200 has an IF construction that makes it change the true focal lenght according to the focusing distance. You barely notice that at 28, but zooming to 200 at the minimum distance of focus it seems to have a 28-135 with a huge zooming stroke from 100 to 135... I'm sure the focusing distance affects the optical performance but I'm not so ackowledged to answer to your question... Sorry. Do you know if the Sigma is a IF lens? I seem to recal it is not, so all my rambling is useless... Gianfranco ===== Send your FREE holiday greetings online! http://greetings.yahoo.com - 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 .
