It is my understanding it stands for "focal length". In a 100mm f/2.8 lens you would divide 100mm by a factor of 2.8 (focal_length/2.8) to get 35.71428mm as the maximum aperture opening diameter. As has been argued here before "f/stop" and "aperture" are two very different things. A 100mm f/2.8 and a 50mm f/2.8 have the same maximum "f/stop" (ratio) but the 50mm "aperture" (actual opening size) is 1/2 the diameter of the 100mm. So f/2.8 isn't an aperture value but a ratio, the higher the ratio (100/2.8 being higher than 100/4.0) the "brighter" (larger maximum aperture) the lens. However, <grin> since a 50mm f/2.0 lens (25mm aperture) and a 100mm f/2.0 lens (50mm aperture) produce the same result in exposure at the film plane (for a constant shutter speed) the two terms tend to get used as meaning the same. It gets more involved in true "telephoto" lenses where lense is physically shorter than the focal length. The "actual" aperture size may or may not follow the above rule, now it becomes an "effective" aperture size. The important fact is that if the proper exposure is 1/125th second at f/5.6 then this holds for any lens focal length or design.
don <whew!> > -----Original Message----- > From: Anders Hultman [mailto:[EMAIL PROTECTED] > Sent: Sunday, September 26, 2004 6:17 AM > To: [EMAIL PROTECTED] > Subject: The meaning of f > > > There have been a discussion on another list about what the "f" in the > aperture value means, like when one say "f2.8" or such. > > Different theories have been put forward. Anyone here who has an > authoritive answer? > > anders > ------------------------- > http://anders.hultman.nu/ > med dagens bild och allt! >
