Re: Ques. about Elamar 90/4 LTM

["Peter Jon White" <pjwhite@tiac.net>]

> Hi,
> I just bought an Elmar 90/4,LTM, sn 15456, and I wanted to ask a few
> questions about it.  Would anybody know when it was manufactured.  I
> assume that the lens is uncoated but when it is put under a light, I
> can count six reflections; the 2 top ones have a very light purplish
> tinge to them and the penultimate one has a slight bluish tinge to
> it.  Are the colours due to a coating or to some sort of optical
> phenomenon?

The colors are the result of anti-reflective coating.

  Why do I see six reflections in the lens; is it because
> there are 6 pieces of glass in the lens?

90mm Elmars had either three or four elements, depending on the age 
of the lens. The reflections are from the surface of the glass 
elements.
> 
> Other questions:
> 
> The f stops are numbered as follows: 4/4.5/6.3/9/12.5/18/25/36.  How
> do they relate to modern f stops?

They are calculated in the same way, so if you need, say, f 8, just 
dial between f 6.3 and f 9.

> The top third of the barrel, containg the glass and aperture blades,
> unscrews from the bottom 2/3 of the barrel.  This part is hollow and
> contains the focusing ring.  Does this capacity to unscrew the glass
> from the rest of the barrel serve some function?

Yes. It allows the lens to be used with the Visoflex reflex 
viewfinder.

  I was thinking
> that maybe one could screw the glass part unto a shorter or longer
> barrel in order to get a different focal length?

Focal length is normally calculated at infinity. In order to change 
the focal length of a lens, you need to change to optical formula of 
the lens, in other words, you need a different lens.

  As an 
alternative,
> I was thinking that the glass part with the aperture blades could be
> used as an enlarger lens.

With the proper adapter, it could, though the lens may not be ideally 
suited for that. Enlarging lenses need to be designed to focus a flat 
plane onto a flat plane. Taking lenses don't require such correction, 
unless they are to be used for copy work.



Peter Jon White