Dwarf
> How does the mm designation of a lens relate to the focal length of the
> lens? I saw from a FAQ that a 35mm lens means that the image of a distant
> object is the same size as through a pinhole 35mm mm from the film. My
> question is, what does this mean? Is that the same as the focal length?
> Is it twice the focal length? Help!
Same as FL... The complications set in with closer than infinity focus, where
the lens is considered to have an unchanged FL, but the distance from the
optical center of the lens and the imaging surface has changed. At the same-size
representation of the image as the subject (1:1), the distance of the lens center
(simple lens...) from the imaging surface would be twice the FL, if I got it right...;-)
BTW, there are assymetrical lens designs that can place the diaphragm of the
lens considerably ahead of or behind the theoretical optical center. This can be
confusing with SLR telephoto and retrofocus design lenses...
> I'm a mechanical engineer trying to select a lens for an experiment. We
> need to resolve a body approx. 1 cm dia from one meter away. I understand
> *basic* ray tracing but don't know how to go from focal length to mm size.
Sounds like a pretty low-resolution requirement - maybe just a pinhole of
appropriate size would work, unless you need greater light transmission...
Hope This Helps
David Ruether - http://www.fcinet.com/ruether