Hallo Bill! The front element of a 50 mm f/1.4 lens has bigger diameter than a 50 mm f/4 lens. (Am I correct?) Therefore the preliminary amount of light energy reaching an imaginary plane or film behind the lens per unit time per unit area is more than a 50 mm f/4 lens as it has a smaller front element. When it is stopped down to f/4, the amount of light reaching the film plane is more than 50 mm f/4 lens. The above explanation depends only on one point that the front element of a 50 mm f/1.4 lens has bigger diameter than 50 mm f/4 lens, if it is at all correct.
However, you are absolutely right that f/4 is still f/4 or in other words, the diameter of the aperture at f/4 is identical in both the cases and therefore, the light has to pass through the holes of identical diameters but what I mean to say is that the intensity of light in 50 mm f/1.4 lens is more than a 50 mm f/4 lens. Please correct me, if I am wrong anywhere. Many thanks for your comments. With best regards, Ayash Kanto. On Mon, 1 Jul 2002, Bill D. Casselberry wrote: > Ayash Kanto Mukherjee 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? I feel that second lens has more light gathering power > > therefore it will give high shutter speed. Am I correct? > > No - they'd be the same since f4 is f4 no matter the lens, etc. > It is possible that one may have some indistinguishable difference > due to light transmission efficieny, but it is doubtful that it > would be significant enough to register except using extremely > small latitude emulsions. I suspect no auto exposure camera system > would be sensitive enough to detect this and change the shutter speed. > > > Bill > > --------------------------------------------------------- > Bill D. Casselberry ; Photography on the Oregon Coast > > http://www.orednet.org/~bcasselb > [EMAIL PROTECTED] > --------------------------------------------------------- > - > 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 .
