# Re: [Rollei] Suplementary lenses, compound system

• Subject: Re: [Rollei] Suplementary lenses, compound system
• From: Richard Knoppow <dickburk >
• Date: Thu, 10 Jan 2002 01:27:27 -0800
• References: <3.0.6.32.20020109221150.008c5ea0 >

```At 10:10 AM 01/10/2002 +0100, you wrote:
> From Richard K.:
>
>>  The formula for the combined focal length of two lenses is:
>>
>> F = f1*f2 / f1 + f2 - d
>>
>>   Where f1 and f2 are the focal lengths of the two lenses and d is the
>> distance between them.
>
>Hm... Richard, I am sure that you meant :
>
>F = (f1*f2) / (f1 + f2 - d)
>
>this can also be written as
I did. I didn't think the brackets were necessary but they probably
prevent confusion.
I wish I were better at writing algebraic stuff in ascii.

If you want to find the principle points of a lens it can be done with
simple equipment although a real optical bench is a convenience.
The procedure is autocollimating. It will measure the focal length and
find the principle points.
Mount the lens in a view camera. Make a small light source, a pencil
flashlight will do. At the simplest place a mirror over the front of the
lens. A first surface mirror is desirable but a plain shaving mirror is
good enough. Place the flashlight against the ground glass near but not at
the center. The image of the light spot will be reflected back through the
lens by the mirror. Focus for sharpest image. This gives you the exact
infinity focus point of the lens. Its a good way of setting the infinty
stops on a press camers.
Now, to find the focal length focus for a 1:1 image of something. A small
ruler works well because you can place a similar small ruler on the ground
glass for comparison. The difference in bellows extension is the focal
length. Now, if the lens is returned to the infinity focus position and a
ruler used to measure off one focal length toward the lens from the ground
glass, its lens end will be exactly at the second principle point. To find
the first or front principle point turn the lens around in the mount, again
focus for infinity using the autocollimation technique and again lay off
one focal length toward the lens. You now have the focal length, both
principle points and the back focus of the lens.
For symmetrical or nearly symmetrical lenses the principle points will be
about one third the distance from the apexes of the lens toward the center.
i.e., if the lens barrel, or rather maximum distance from front apex to
rear apex is three inches each principle point will be about one inch in
from the front or back of the lens.
Of course telephoto and reverse telephoto lenses vary considerably from
this, both principle points being outside the lens. Single cells from
convertible lenses generally have one principle point, usually the one on
the convex side, lying outside the lens.
- ----
Richard Knoppow
Los Angeles, CA, USA
dickburk

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