Archived posting to the Leica Users Group, 1997/02/18
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]> I may be wrong, but I always thought that you have to divide the diameter of > the aperture (or the diameter of the hole, in the case of a pinhole) by the > focal length. > > Regards, > > - Juan Greetings, That's correct. And it's easy enough to prove. Take a metric ruler and hold it across the front of a 50mm lens with the aperture at f:2. The opening you see will measure almost exactly 25mm. A 1mm pinhole, at a focal length of 50mm would have an aperture of f:50. A Noctilux, with a focal length of 50mm and an aperture of f:1 has a diaphragm opening 50mm in diameter, not radius. If someone has a system that measures by radius, it's not the system being used by all lens makers I'm aware of. Note that with a pinhole, it's the actual diaphragm opening that's measured, whereas on a lens its the size of the opening you can see that's measured. If you took the lens apart the actual size of the diaphragm would likely be smaller. Peter Jon White