Hi Yves! Your explanation is absolutely clear to me. So, it is the diaphragm which blocks of the extra amount of light collected by lenses with bigger front element.
Yeah! I liked the pinch of salt. I am very much thankful to you and Bill. Now, I am in a position to arrive at the second stage of the drama. I was watching world cup football on TV and there I saw that all the photographers are equipped with small f-number lenses, seemed to be 300 mm f/2.8 kind. They must be shooting at shutter speed of 1/125 th of second in order to stop the action. Now the light in an artificially illuminated stadium is too low. The solution to the problem is to use a fast film and a steady tripod since the focal length is too large to hand hold the set-up. I shall like to know what aperture do they use with what film speed? Do they use those big lenses to take an advantage of f/2.8 or do they always shoot at wide open apertures? (I don't think so as the photographs appear quite sharp with good depth of field (3 m) in the sports magazine) I can put the question in another manner. Suppose I have a 300 mm f/5.6 lens. What film speed should I use in order to stop the action as well as attaining good depth of focus (say, 4 m) in an artificially illuminated stadium? I apologize for this kind of complicated question. I am just inquisitive, that's all. Many thanks for explanation. It cleared a lot of doubts. With best regards, Ayash. On Mon, 1 Jul 2002, Yves Caudano wrote: > At 23:11 1/7/02 +0530, you wrote: > > Hi again! > > You probably sent this mail before receiving my previous answer: I hope this will >clarify it anyway. > > >The front element of a 50 mm f/1.4 lens has bigger diameter than a 50 mm > >f/4 lens. (Am I correct?) > > Yes > > > 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. > > If the diaphragm is wide open, yes. No, if both lenses are set to the same f-stop. > > > > >When it is stopped down to f/4, the amount > >of light reaching the film plane is more than 50 mm f/4 lens. > > No, because you will close down the diaphragm of the F1.4 lens so that the same >amount of ligth will arrive on the film than with the F4 lens with its diaphragm wide >open. The additional, outer rays, allowed by the larger front elements of the F1.4 >lens are blocked by the diaphragm, so that, eventually, the same amount of light >reaches the film. > > >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. > > It depends also on the size of the diaphragm. > > >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 > > As a said in my previous mail, the diameter of the aperture at f4 may vary between >lenses (especially of different focal length). However, the amount of light reaching >the film at f4 is identical from lens to lens (by definition of f-stops, and this is >why they are useful!). > > >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. > > I am writing the following quickly, so take it with a pinch of salt: if the F1.4 has >the same design than the f4 lens and just has larger front elements, I indeed would >guess that the diaphragm opening of the F1.4 lens at F4 should be the same than the >size of the diaphragm of the F4 lens wide open. In other words, I expect that, in >that *particular* case, the diaphragm of both lens would have the same diameter when >the same amount of light reaches the film. I may be wrong though. > > However, this would be definitely true for a very simple lens consisting of a single >element: in that case, the amount of light reaching the film depends only on the >diaphragm size and not on the (larger) lens diameter, since the diaphragm blocks all >the outer rays and lets only the rays coming from the lens center in. Closing the >diaphragm behind a large lens is then equivalent to take a lens with a smaller >diameter. > > >Many thanks for your comments. > > You are welcome. > > Yves > > > ------------------------------------------------- > Dr. Yves Caudano > Laboratoire LASMOS > D�partement de Physique > Facult�s 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 .
