Re: Optics question (not alt)

From: Richard Knoppow ^lt;[email protected]>
Date: 03/10/05-01:06:46 AM Z
Message-id: <003d01c5253f$bc0dde40$66fd5142@VALUED20606295>

----- Original Message -----
From: "joachim oppenheimer" <>
To: <>
Cc: "Dan Burkholder" <>
Sent: Wednesday, March 09, 2005 5:01 PM
Subject: RE: Optics question (not alt)

>I don't know if this helps: I have a copy of Richard W.
>St.Clair, A.R.P.S.,
> "Photographic Lenses and shutters" (1940) page 55, and I
> quote: QUOTE:
> Determining effective aperture: In the case of a single
> lens, with the iris
> placed in front of the lens, the effective aperture is the
> diameter of this
> stop opening. All of the light that enters must enter
> through this aperture.
> But, in the case of multi-element lenses the light beam
> entering the front
> lens is slightly converged or condensed by the front lens
> element before it
> reaches the iris which, in this type of objective, is
> placed between the
> front and the rear elements. The diameter of the beam is
> now less at the
> iris than at the point of entry and the whole matter is
> rather uncertain.
> END QUOTE. If I understand your question correctly, and
> since virtually all
> modern lenses are compound lenses, I would conclude that
> the image size (on
> film or sensor) will be smaller but in a unpredictable way
> depending on the
> specifics of the lenses and related factors (focal
> distances, etc.) Joe
   The location of the stop does not affect the size of the
image. However, the _effective_ size of the stop _is_
affected, and this is what the above is trying to say.
   The light gathering power of a lens is determined by the
size of the entrance pupil. The entrance pupil is the
_image_ of the stop as seen from the front of the lens.
There is another pupil on the other side of the lens, called
the exit pupil. The position and size of the pupils are
determined by the power of the lens elements inbetween them
and the outside.
   Where the actual physical stop is outside the lens, as
when a convertible lens cell is used on the back of a barrel
or shutter, the position and size is the actual size and
position of the stop. When there is a lens between it and
the outside world the power of the lens determines the size
and position. For a negative lens the pupil will appear to
be smaller than the stop and further away. For a positive
lens it will appear to be closer and larger. For instance, a
half Dagor used behing the stop has a speed of about f/13,
when its put in front of the stop the speed is closer to
f/11.5. Not a big difference, but a difference.
   To measure the location of the stop you need a camera
which will focus at a very close distance. First, focus it
on a referece surface, the rim of the cell will do. Then
_move the camera_ so that it now focuses on the stop. The
distance and direction it moved will tell you where the stop
is in relation to the reference surface.
   To measure the size of the pupil you need to determine
the exact focal plane for infinity focus. This is easily
done by autocollimating with a small mirror. I've posted
instructions for this before, I think to this list. Once the
focal plane is found place a small light source there like a
flashlight behind a small hole. Now place something
translucent over the lens. Ground glass is great but
ordinary bond paper works OK. There will be a circle of
light projected onto the paper. This is the image of the
entrance pupil and its diameter is the _effective_ diameter
of the stop.
  With the help of the small mirror (a plane shaving mirror
will do) a small flashlight and a view camera as an optical
bench it is possible to measure the focal length, pupil
locations, principal plane locations, and calibrate the
f/stops of any lens.

Richard Knoppow
Los Angeles, CA, USA 
Received on Thu Mar 10 01:06:59 2005

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