[LargeFormat] calculating aperture scale - an easy way?

[Richard Knoppow]

----- Original Message ----- 
From: "Timothy Atherton" <tim at KairosPhoto.com>
To: "Richard Knoppow" <dickburk at ix.netcom.com>; "f32 Large 
Format Photography Mail List" <largeformat at f32.net>
Sent: Saturday, February 04, 2006 9:17 AM
Subject: [LargeFormat] calculating aperture scale - an easy 
way?


>I vaguely remember Richard writing on this once, but can't 
>find it?
>
> Is there an easy way to calculate an aperture scale for a 
> shutter/lens?  I
> have my 210mm Kowa Graphic in a Copal #1 shutter with an 
> appropriately
> marked aperture scale.  Say I want to put a 150mm Kowa 
> Graphic in to try it
> for a while - is there
> a way to come up with a scale without sending the whole 
> thing off to
> Grimes or somewhere? (I'm not talking about the actual 
> little piece of
> metal for the scale itself - I'll just use a piece of tape 
> or some such
> and mark it - rather the measurements) thanks
>
>
> timothy atherton

    Since the two lenses are of the same type you can 
probably just scale the aperture. You must devide the 
original scale by the ratio of the focal lengths.
    The post you are thinking of is a description of how to 
measure the stops for an unknown lens. You can apply it here 
if you choose.
    There are two parts. The first is measuring the focal 
length, the second is measuring the size of the entrance 
pupil, which is the _effective_ size of the stop. Unless you 
need a very accurate focal length measurement you can assume 
the marked FL is correct. I give both measurements because 
they are done with the same tools.
    To measure the size of the entrance pupil you need a 
card with a small hole in it and a source of light, a small 
flashlight will do. The card is placed at exactly the focal 
plane for infinity focus. This is easy to do using a small 
flat mirror to autocollimate the lens. Place the mirror over 
the lens and adjust the card so that the spot of light 
reflected back to it is focused as sharply as possible. Get 
the spot close to the illuminated hole. Of course, you won't 
see it if its _exactly_ at the hole.
   Now, the lens is focused at infinity. Remove the mirror 
and replace it with a translucent screen. Ground glass is 
best but a sheet of thin paper will do fine. The circle of 
light projected onto the paper is an image of the entrance 
pupil. Measure its diameter and devide the focal length by 
this diameter to get the stops. The diaphragm is affected by 
the lens in front of it so the effecive size is not always 
the same as the physical size. In any case, this method 
allows measuring the effective size of the stop without any 
disassembly of the lens.
   To measure the focal length you need one additional 
measurement. Mark the position of infinity focus at some 
convenient reference point. The exact point does not matter 
because it is the shift of the lens that will be measured.
   Next focus on some convenient object to obtain an exact 
same size image. The distance the lens moves from its 
infinity focus position is exactly one focal length. This 
can be checked by measuring the total distance between image 
and object at 1:1, it will be exactly four times the focal 
length.
   If you want to know the locations of the principle points 
you can find them by focusing the lens at infinity and 
measuring one focal length back from the image toward the 
lens. When the lens is facing in its normal direction you 
will be measuring the 2nd or rear principal point, when the 
lens is reversed (front facing the image) you will be 
finding the first, or front principal point.
   The locations of the entrance and exit pupils can be 
found by using a camera which will focus at very close 
distances as a measuring tool. First, focus it on the rim of 
the lens cell or other convenient point. Then move it so 
that the iris is in sharp focus. The direction and distance 
the camera moves will give you the location of the pupil in 
relation to the reference point.
   While this is all easier to measure with a proper optical 
bench decent measurements can be made with a view camera and 
simple accessories.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk at ix.netcom.com