Can front-cell focusing lenses be unit-focused instead when adapted?
Asked 10/1/2019
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Some vintage triplet and Tessar-type lenses, especially from viewfinder cameras, focus by moving only the front element or front cell. When adapting one of these lenses to another camera, does image quality change depending on how focusing is done? For example, is it better to:
- set the front cell to infinity and focus by moving the whole lens with a helicoid,
- unit-focus the whole lens with the front cell set to some other distance,
- preserve the original flange distance so the front-cell focusing works as designed,
- or mix both methods as convenient?
I'm mainly asking about optical performance with simple vintage designs like triplets and Tessars.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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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:
Originally by user75526. Source · Licensed CC BY-SA 4.0
user75526
6y ago
0
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Based on the shared experience, the safest choice is to set the front cell to infinity and focus by moving the entire lens. One user who adapted a Steinheil Cassarit 50mm f/2.8 found it was sharpest at all distances when used that way.
That result makes optical sense for simple designs like triplets and Tessars. Moving only the front element changes the spacing between elements and can increase aberrations, while unit focusing keeps the lens formula intact and usually preserves image quality better. These older lenses often do not have enough elements to fully correct for the compromises introduced by front-cell focusing.
So, if you are adapting one, unit focusing with the front cell at infinity is likely the best image-quality option. Using the front cell at other settings while also unit focusing will probably be less consistent. Preserving the original front-cell focusing arrangement may work, but it is not likely to outperform simple unit focusing at infinity.
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AI6y ago
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