Archived posting to the Leica Users Group, 2000/10/14

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Subject: Re: [Leica] Enlarging lenses on screw mount camera pt. 2
From: "Jem Kime" <jem.kime@cwcom.net>
Date: Sat, 14 Oct 2000 08:44:57 +0100
References: <20001014035816.15007.qmail@nwcst277.netaddress.usa.net> <039901c03598$83d13180$42fa0b18@alngtn1.va.home.com>

Isaac,
If they are 50mm lenses then you're in luck.
Although we think of most R/F lenses having complicated cams, translating
the distance the lens moves (to achieve focus) to the distance the cam moves
(to show focus), the 50mm is the only one which works 1:1. I.e., for every
millimetre the optical unit moves forward, the cam is moved by the same
amount. So the profile on the rear of the 50mm lens is always flat.
Look at any 50mm lens and then measure (incredibly accurately) the distance
moved and the focus achieved. (e.g. one millimetre = inf.-25') etc. I
measure (very roughly) that there are three millimteres of travel between
inf. and 1m focus.
If you wish to be more precise in marking focus points, far better to find a
true focus screen and critically examine with a 10x magnifier.
I've just remembered you mentioned an 80mm(?) as well. Well, you're on your
own there!
regards,
Jem
- ----- Original Message -----
From: Isaac Crawford <isaacc7@home.net>

> Well, I did a little homework and it turns out that I need 13.2mm and
48.7mm
> of extension to achieve infinity focus with my enlarging lenses. Luckilily
I
> can use some fairly inexpensive extension tubes to get a little beyond
that.
> The question is how much closer from infinity will they be? Is there an
easy
> formula to determine the focusing distance based on the amount of
extension
> past infinity?
>
> Thanx
> Isaac
>

In reply to: Message from s.barrett@usa.net ([Leica] 35mm Summicron-M Chrome For Sale)
Message from "Isaac Crawford" <isaacc7@home.net> ([Leica] Enlarging lenses on screw mount camera pt. 2)