Archived posting to the Leica Users Group, 1997/02/21

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Subject: Aperture calculations
From: henningw@portal.ca (Henning J. Wulff)
Date: Fri, 21 Feb 1997 12:06:24 -0800 (PST)

Thomas Myro wrote:

>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.

I can't take it any more!!

The f/ number is an 'aperture ratio', and that ratio is the rear focal
distance of the lens divided by the apparent _diameter_ of the lens
aperture (opening) seen from the distance the lens is focussed on. The f/
number stamped on a lens is intended to represent the aperture ratio when
the lens is focussed on infinity. Note that the aperture in question is the
'entrance pupil'. I'm afraid that were Thomas' explanation correct, lenses
such as Canon's and Nikon's 600/4 would not exist, as their exit pupils
would have to be 150mm across, which I believe is somewhat larger than the
lens mount :-). Light can indeed be 'crammed' into a smaller area, and that
is exactly what lenses are designed to do. If they could not concentrate
light, all of optics would only be useful for producing rainbows through
diffraction. Curved surfaces allow all of the fine optics we know to work,
and concentration (and spreading) of light is what it is all about.

The f/ number of a pinhole camera is the distance the pinhole is from the
film (or paper, or whatever) surface divided by the diameter of the
pinhole, treating the pinhole as a theoretical surface, ie., one with no
thickness.

BTW, my emphatic vote is to keep the LUGnutters discussions here on this
list, rather than a newsgroup. This is a fine group, and as long as some
sense of humour prevails, I enjoy all of it, including the somewhat
off-the-topic pronouncements of Oddmund. Other points of view is what I
subscribe to mailing lists for. My own I know, and while it is pleasant to
have others agree with you, divergent opinions are a lot more fun.


   *
  /|\     Henning J. Wulff
 /###\      Architecture
 |[ ]|   henningw@portal.ca