What types of lenses are most likely to show focus shift when stopping down?
Asked 1/12/2014
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2 answers
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Some lenses shift the plane of focus rearward as the aperture is closed, often due to uncorrected or under-corrected spherical aberration. This tends to be discussed most with fast primes and is often most noticeable at close focus distances and mid-to-wide apertures.
What lens design traits make focus shift more likely? Are there broad patterns—such as fast apertures, older optical formulas, or lenses tuned for smoother bokeh—that suggest a lens may show this behavior?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
5
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
Originally by user24260. Source · Licensed CC BY-SA 4.0
user24260
12y ago
0
Generated from our catalog & community — verify before relying on it.
There isn’t a single reliable design feature that guarantees focus shift, but some patterns are common.
Lenses most likely to show it are fast primes, especially designs that leave some spherical aberration uncorrected to favor smoother bokeh or a more “classic” rendering. Older optical formulas are more likely candidates. The effect is often most noticeable at close focus distances and at mid-to-wide apertures.
Aspherical elements and floating elements can help reduce the chance of focus shift, because they improve correction of spherical aberration across focus distances. But their absence does not mean a lens will shift focus; some designs simply don’t need them.
Focus shift is not limited to one focal length range. It can appear in wide-angle, normal, or telephoto lenses, though it may be easier to notice with longer lenses because shallower apparent depth of field makes the shift more visible.
So the best practical rule is: be most cautious with fast primes, particularly older or deliberately characterful designs, and verify behavior with real tests rather than assuming from specs alone.
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UniqueBot
AI12y ago