Re: Elmar 90mm f4

[Dennis Painter <dwp@deltanet.com>]

Steve Kobrin wrote:
> 
> The Elmar 90 f4 was made in both screw mount and M versions.   I have an
> Elmar M made about 1960 or so and wonder if it is identical mechanically to
> the screw mount versions.  If it is, could I then unscrew the lens from the
> barrel and mount it in a barrel designed for a scew mount camera?  Would it
> focus properly on a IIIF?
> 
> If the answer to both questions is yes, I could then look for a screw mount
> Elmar with bad glass and a decent barrel and then use the same "lens" on
> both my M3 and IIIF, saving myself some money and taking advantage of that
> fact that the lens I have is in relatively good shape.
> 
> This sounds hair-brained and I suspect there are eighteen reasons why it
> cannot be done.  Anyone know what they are??
> 
> Steve

Steve,

In general they are not interchangable.  Leica measured the actual focal
length of each lens and used a focua mount with a screw pitch that
matched the focal length of the lens head.  In addition they may have
'hand' touched up the focus cam.  Others may comment on the focus cam, I
am not sure if it was done on all lenses.

What this means is that if the actual focal lenght of your 90mm was, asy
90.5 mm and you mated it to a focus mount with a screw pitch for a 89.0
mm lens it will only focus correctly at infinity.  This is because the
movement of the lens head from infinity to focus on a closer object
varies with the focal length of the lens.

I hope this is clear and answers your question.

Dennis

- -- 
Experience is a tough teacher. It gives the test before the lesson.
- -unknown