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 .
