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Some lenses, because of a design that includes uncorrected spherical aberration, shift their plane of focus backwards as the aperture is narrowed. This is most apparent at close focusing distances and mid-to-wide apertures, but is not a defect and should not be subject to 'sample variation', yet reputable reviewers will either disagree about its presence or not test for it at all.

Which design features or lens characteristics are most likely to indicate a lens that will exhibit focus shift from un/under-corrected spherical aberration? Which contemporary lenses are known, either through direct experience or reputational consensus, to show this characteristic?

For example, from direct experience I know the Zeiss ZM C-Sonnar 1,5/50 does this, and the Canon EF 50/1.2L also has a reputation for having mild focus shift. This previous question touches on a potential cause of this behaviour with Zeiss primes for SLRs, but doesn't delve into details: Focus shift when stopping down on Zeiss Primes?

  • Are you looking for a list? Stack Exchange isn't a very good match for that kind of thing. – mattdm Jan 12 '14 at 11:12
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    Thanks, @mattdm, I've edited the question to broaden it beyond list-making and to incorporate the reference to the earlier question that was previously in my comment. Please let me know if my question can still be improved. – mpr Jan 12 '14 at 11:56
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After some reading, this is what I've found:

There's no guaranteed indicator of which lenses will show focus shift, but fast primes, especially lenses optimized for smooth out-of-focus blur ('bokeh'), are the most likely to show it. Older designs or those intentionally pursuing a 'classic' appearance are particularly likely.

Single aspects of lens construction aren't a strong indicator. Aspherical and floating lens elements reduce the likelihood of un/under-corrected spherical aberration, but their absence doesn't suggest its presence – the lens design may simply not require them. Focus shift can occur with wide angle lenses as well as normal or longer focal lengths, although narrower apparent depth of field from the higher magnification of a long lens can make the effect more visible.

Despite being a fact of lens behaviour, even reading reviews isn't a reliable indicator of focus shift. Reputable sources can disagree on this, even when they're testing under conditions most likely to show its effects. Of course those reviews that test for spherical aberration and/or focus shift directly are particularly useful predictors.

Given that I haven't been able to produce a conclusive answer, here are some of the sources that I found useful, offered in the hopes that others can glean more than I did:

http://photographylife.com/what-is-focus-shift

http://diglloyd.com/articles/Focus/FocusShift.html

http://lavidaleica.com/content/lens-primer-advanced-topics

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