Hallo Yves! Just a few minutes ago, I replied to Bill.
You are absolutely right in your point. I know that quite well. The f-number is defined as the ratio of focal length to the diameter of the aperture (Am I correct?). But I am talking about the preliminary intensity of light reaching the film plane because of the wider diameter of the front element in the case of 50 mm f/1.4. Your reply is of course comprehensible. Many thanks for your reply. With best regards, Ayash On Mon, 1 Jul 2002, Yves Caudano wrote: > At 20:56 1/7/02 +0530, you wrote: > > >I have some doubts regarding lenses with small f-numbers. Suppose you have > >two lenses, one is 50 mm f/4 and the other is 50 mm f/1.4. If, I stop > >down the second lens to f/4, which one will give faster shutter speed for > >correct exposure? > > The correct exposure will be obtained with the same shutter speed for both lenses. Actually, that same shutter speed should even give the correct exposure for any lens [*]. This is because f-stops are defined so that this property holds. > > As a result, for identical f-stops, the actual diameter of the diaphragm will depend on the focal length and the lens design. > > To go back to your comparison between the two 50 mm lenses, I expect the optical >quality of the F1.4 lens to be better at f4 than the F4 lens, though (since you are >not pushing your lens to its "limits," especially regarding vignetting). > > [*] assuming infinity focus and (as mentioned earlier by Bill Casselberry) >negligible transmission losses. > > Hope this helps (and is comprehensible), > > Yves > > > ------------------------------------------------- > Dr. Yves Caudano > Laboratoire LASMOS > Département de Physique > Facultés Universitaires Notre-Dame de la Paix > 61 Rue de Bruxelles > B-5000 Namur > Belgium > > [EMAIL PROTECTED] > > tel : + 32 (0)81 72 5487 > fax : 4707 > > URL : http://www.scf.fundp.ac.be/~ycaudano/ > > Lasmos laboratory URL : > http://www.fundp.ac.be/sciences/physique/lasmos/ > - > 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 . - 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 .
