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Some vintage tessar or triplet type lenses (eg such intended for viewfinder cameras) implement focusing by merely moving the front element or front cell.

When adapting such lenses to other cameras, is there an image quality difference between:

  • unit focusing the lens with the front cell set to the infinity mark
  • unit focusing the lens with the front cell set any other way
  • actually aligning the flange distance so the front cell focusing can be correctly used
  • mixing and matching as is convenient

?

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I have previously disassembled and adapted one such lens – Steinheil-Munchin Cassarit 50/2.8. At all distances, it was sharpest when the front element was set to infinity and the entire lens was unit focused with a helicoid adapter.

I would expect similar results with similar lenses because there aren't enough elements to correct aberrations when elements are moved around. Unit focusing lenses often have fewer than eight elements, but modern non-unit focusing lenses often have 14+ elements. The focusing groups in lenses I've examined often have three or more elements.

Here are a couple images taken with the aforementioned lens:

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  • Actually have that lens in M39/Braun mount, but it is unit focusing (and for a non-prewar triplet, rather weak in the corners even on APS-C I think)... – rackandboneman Oct 5 at 15:04
  • Mine had an M42 mount. I'm pretty sure it's the lens I was thinking about. I don't have it any more. – xiota Oct 5 at 16:52

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