Summicron 35mm hood query + old LTM lens question

[Prof-K]

Hello,

 

I just bought a <b>type-3</b> Summicron 35mm/2 lens (so far my costliest lens).

Can you tell me if the square hood for the 4th-version 35'cron will work with

this lens?

Can I use it on top of a 39mm screw-on filter? (I already know about the

original hood for the 3rd version).

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Also, another question about old LTM lenses: can anyone tell me what the number

on the back of the infinity catch/lock means (e.g. on the Summar)?

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Searched the forum w/no luck. Thanks in advance.

<br>

Shakil

[sumo_kun]

Not sure about the hood but for the number, on my summar, there is what looks like an 8. This could be a sideways infinity, meaning infinity lock? I don't have any other summars to compare so I don't know if I'm just lucky with the 8.

[john_shriver]

The number on the back of the infinity catch lock indicates what focal length group the lens is in, and thus what the pitch of the focusing threads and the shape of the cam is. It turns out to be very hard to control the exact focal length of lenses, because it is hard to control the thickness of each lens element. So Leica picks compatible sets of elements (in terms of thickness), assembles them into a lens unit, and measures the focal length. They are then classed into focal length groups, each of which has a range of a few tenths of a millimeter. There are focusing mounts made that are calibrated for the target focal length of each group, the lens is placed in that mount, and the lens properly collimated for infinity focus (a shim).

 

The same thing happens in SLR lenses, but they don't make different focusing mounts. So the focusing scales are not as accurate there. But they also have a selected shim for infinity focus.

 

Erwin Puts' "Leica Lens Compendium" book has tables in the back giving the mapping from these numbers to the focal length range for various lens types.

 

I think the Summar uses the same group coding as the Elmar, so the target focal length of group 8 is 51.9mm. That group covers focal lengths from 51.75mm to 52.05mm. (However, the chart in the book may be for newer lenses than a Summar.)

 

The important implication of all this is that nitwits who swap lens cells and focusing mounts willy-nilly make mutt lenses that will never focus accurately. This is why the serial number is often marked or engraved in the focusing mount on lenses where they are easy to separate. Also, Leica scratched the measured focal length on the side of the lens cell on my LTM collapsible Summicron.

[Prof-K]

Thanks John, Chuk. So it has the same meaning as in the rigid summicron (although the number is not under the infinity catch on the summicron). Not sure how I forgot about that.

 

Any help on the hood will be appreciated.

 

Cheers,

Shakil

[paul t]

I used the square hood on the V2 and it was fine, but for the fact a small tab which lined up

the hood, obstructed the focusing 'flag'. The V3 has no flag so I foresee no problem

whatsoever.

[Prof-K]

Thanks a lot, Paul.