What does a "correctly adjusted" lens mean?

[micromoogman]

Hi all,
I just bought what I thought were two garbage 50/3,5 Elmars, $50 each, prewar uncoated...but when I got them they were really really clean and in very good shape. I'm just about to shoot a roll.
But I wonder what some people talk about when they say "correctly adjusted" about some lenses. I don't get it, the glass must be in their right positions, the barrel fixed in out position...what could a adjustment do to it? Are the glass shimmed or in any way adjustable? I have in most cases seen this in the FSU forum, but also refering to Leica lenses as Elmar.

 

[jlw]

A lens has to be the correct distance from the film plane to produce a sharp image. With SLR cameras, slight differences don't matter because you're focusing by viewing through the lens. With an RF camera, though, correct spacing is critical because otherwise the focus point of the lens won't agree with the focus point recommended by the rangefinder.

Because quality control of FSU lenses sometimes wasn't very tight, it's not uncommon to get an FSU lens that's set for a spacing that doesn't exactly match a specific FSU camera. Some FSU buffs have learned how to adjust the shims, screws, etc., in the internal construction of the lens to give it the best possible match to the rangefinder calibration of their specific camera. I think that's usually what they mean when they say "correctly adjusted."

(To be fair, it's not just FSU lenses that sometimes need this. Even though the flange-to-film distance of LTM cameras is supposed to be standardized, different manufacturers made different assumptions about film thickness, film-gate clearance, mounting torque, etc., which can lead to variations. For example, it's not uncommon for the well-liked Canon 50/1.2 lens to focus slightly in front of the rangefinder's target distance when used on a non-Canon camera; I had to sand about 0.02mm off my Canon 50/1.2's "collimation shim" to get it into exact agreement with the rangefinder on my R-D 1.)

 

[Nikon Bob]

The only thing I can think of regarding pre war Elmars is that the very early ones were matched to individual cameras and not readily interchangable with other bodies. Some were also non RF coupled. I think this had to do with the film to flange distance not being standard on all bodies. Later pre war Elmars were interchangable/ RF coupled and I believe that those lenses were marked with a 0 on the lens flange. If yours are in good shape and haze free you will be surprised at how good they still are. Just watch out for situations that cause flare. Lets see some results and comments on your Elmars.

Bob

 

[tmessenger]

Even variations of the same lens model can be off. My chrome 50/1.8 Canon focused correctly on my Epson R-D1 but my black and silver version of the same lens back focused and required .001" shim to correct. The bottom line is the rangefinder and lens have to agree and this is a mechanical relationship.

Tim

 

[Kim Coxon]

There are in fact three problems. The first 2 have already been mentioned. Early Leicas had slightly non standard flange/film plane distances and the lenses were mached to the camera. The other 2 affect mainly FSU lenses. The first is the accuracy of the flage/film plane and if is is not correct due to manufacture or "table top" fiddling, then the lens will not focus correctly at infinity even if made for the LTM standard. This is relatively easy the fix.

The last one is much more problimatical to fix. Not only must the lens sit the correct distance from the film but the RF coupling must move the correct amount for a "standard" 50mm lens regardless of it focal length for it to remain correctly focussed throughout the range. The J9s and J3s are especially prone to this problem.

Kim

 

[xayraa33]

the DOF of a 50/3.5 lens works as an advantage here,
a max aperture of say, F 1.5 will certainly be noticed in a poorly adjusted lens.

 

[micromoogman]

Ok, thanks for the answers! I guess the RF must be tested for accuracy as well...
I noticed one odd thing with one of these Elmars, the glass is clear but holding it in some angles towards a light source it shows a small rainbow effect, not like the consistent blueish coating tone, but more like a prism... What is this? It is not visible when looking straight through.

 

[xayraa33]

the rainbow effect could be element separation.
I seen something similar on a Canon SLR lens that received an impact or a blow , like when dropped.