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Why do some lenses shift focus when stopping down? All four combined components of my Staeble Polyplast coffin set (105, 135, 165, 195mm) shift forward about 1/8", while the rear component if used alone (220mm) shifts 1/8" rearward. (PS, thank you Dagor77 for selling me this wonderful little gem.)
Spherical aberrations -- and older glass has more of it than newer glass.
What spherical aberrations mean is that a lens does not focus the rays that enter through the central part of the element to the same point that rays entering the outer part of the element do; usually the outer edges focus the rays shorter than the central part of the element. (Not to be confused with Chromatic aberrations.) Thus as you stop the lens down you "trim" out the outer rays and gain relative concentration on the inner rays, generating a visible focus shift. The problem gets exacerbated when the aperture is not at the optical center of the lens.
Cheers,
To add on to Jack's post: Quite frequently spherical aberrations are due not to design flaws buy to imperfect alignment of the cemented elements. So, if the guilty lens is recemented and aligned correctly much of the focus shift may disappear. Of course, recementing by a professional can be very expensive, and probably isn't worth while but with the rarest of lenses.
I had a junker 12" Turner-Reich convertible which exhibited really severe focus shift. A friend (who worked in a university optics lab and had access to the correct tools and uv curing glue) and I recemented its elements and the shift all but disappeared. Of course, I dropped the lens on a tile floor a few weeks later...