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EXCERPT page containing first few paragraphs. 2023-03-25 13:12:34

Aspheric “Onion Ring” Bokeh

The process of grinding aspheric elements can leave “fingerprints” which can be seen with a defocused point source. While it can show up and look rather strange, it is very unlikely to ever show up and be of concern in a “real” photo.

According to Brian Caldwell, optical designer of the Coastal Optics 60mm f/4 UV-VIS-IR APO macro:

The "tree rings" are almost certainly due to residual ripple on an aspheric surface. I would guess that Leica has relied a little too much on automated polishing equipment. If you try doing a knife edge test (put a razor blade near the focal point of a point object) the rippled appearance should become much more obvious.

This phenonomenon is also noted on page 32 of Depth of Field and Bokeh.

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Concentric rings, likely due to aspheric lens element grinding
Leica 70mm f/2.5 Summarit-S ASPH

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