Re: Pinhole f number

[photology@juno.com (Thomas P Myro)]

>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
>
Good Day!

In reality, it's the rear element size that determines the maximum
aperture of an objective--the opening of the rear element-- Think about
it,  how can light from something that appears to be 50mm wide cram
through an opening that is only 35mm or so wide-- the "appearance" of the
aperture size by looking through the front element is a coincidence, and
not indicative of anything, evidently.

As to radius vs diameter, argue with Dr. Abbe, not me.

The light pencil-rays, not yet having passed the focal point, still
converging, are limited in aperture by the opening of the rear element.


Sincerely,

Thomas P. Myro
Photology@juno.com