What would these lenses be used for?

Discussion in 'Classic Manual Cameras' started by alan_anderson, Apr 28, 2021.

  1. Sorry, didn't make that clear above.

    Basic optics

    For a simple, single element lens, the focal length of the lens is the distance from the middle of the lens element (nodal point) to the focal plane, so 50mm for a 50mm lens, at infinity. To focus closer, you increase that distance.

    So, the lens barrel for a 50mm lens will be the focal length minus the flange - focal distance (distance from the lens mount to the film), so 4.8mm for a SLR (50-45.2) and 21.2mm for a rangefinder (50-28.8).

    So if you mount a rangefinder lens on an SLR, you are effectively adding a 16.4mm (45.2-28.8) extension tube and will only be able to focus at very close distances. If you mount a SLR lens on a rangefinder, you will be beyond infinity, the focal point will be 16.4mm behind the film plane and it won't work, not to mention the lack of RF coupling.

    This pretty much holds true for a Tessar type lens.

    It all gets a bit more complex with telephoto (nodal point is in front of the physical lens) and retrofocus (nodal point behind the physical lens, closer to the film).

    This is why wide angle lenses for SLRs are retrofocus designs, the mirror prevents the lens being physically close enough to the film for, say, a 28mm focal length.


    Again, if I've made a mistake or over simplified, please correct me.


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    LTM being referred to as M39 irks me somewhat, not only is it the source of all this confusion, but it's simply wrong. The 'M' in M39, M42, M8 specifies a metric (ISO 68-1) thread, but LTM is a Whitworth thread...
     
    ] likes this.
  2. Thanks for the extensive clarification Steve
     
  3. It gets worse when you throw enlarger lenses into the mix, that nearly all use a 39mm thread where physically possible. I'm not sure they were ever 'standardised' to a 1mm or 26TPI thread, but I still have a couple of enlarging lenses that bind slightly when offered up to a 39mm enlarger or bellows flange.

    The ~ 30 micron pitch difference should make absolutely no practical difference to the fit of a thread over six to ten turns. So I can only put it down to a thread profile difference.... or just plain poor machining.

    Also - geek fact - the definition of the imperial inch was only standardised to exactly 25.4mm in 1930. Which is well after Barnack and Leitz settled on their weirdly hybrid 39mm O.D. & 26TPI lens thread. Before that the UK imperial inch was about 23 nanometres shorter. Insignificant in practical terms, since most materials will shrink or grow by more than that with only a small temperature change.
     
    Last edited: May 5, 2021
  4. Thanks for informative discussion. I wonder if the Tessar was made for the Paxette? Photos above.
    Alan.
    Apologies for duplicating pictures.
     
  5. m42dave

    m42dave Dave E.

    Alan, I'm pretty sure your Tessar is for the Paxette. It looks like the one in the link I posted earlier in this thread, and the same as a recent eBay listing.

    The Industar-50 is definitely the SLR version, probably taking 33mm filters. The first two digits of the serial number would indicate year of manufacture. In addition to the early Zenits mentioned, the KMZ Start and Kiev 10/15 could also use these M39 lenses with dedicated adapters.
     
    Last edited: May 5, 2021
  6. As above, Industar is for a Zenit SLR, probably an early Zenit E or B/V, before they switched to M42 (earlier cameras had bare aluminium lenses, not black).

    As to the Tessar, do you have access to a digital mirrorless camera? A cheap LTM to mirrorless adaptor will tell all...
     

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