Found the answer to my own question regarding the Sigma 125 with PAN FOCUS:
*PAN lenses can stop down to f/40 and f/64 and similar tiny apertures.
The problem is they run into Diffraction
<http://medfmt.8k.com/mf/diffraction.html> effects, which make the image
less sharp at f/22, f/32,.. f/64 and so on. The advantage is greater
depth of field, particularly for closer focusing and macrowork. Not a
great tradeoff, it didn't catch on ;-0) [See Roger Hicks and Frances
Schultze The Lens Book]]*
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