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A couple of images in a question about how to create a 'medieval look' appear to have ring-shaped bokeh.

  • Catadioptric (mirror) lenses can cause similarly shaped bokeh, but these images do not look like they were taken from a tremendous distance with a 400-600mm lens (the typical focal length of mirror lenses).

  • Another question describes similar shapes in the viewfinder of a camera, but the recorded image was not affected.

Besides mirror lenses, what else could cause the ring-shaped bokeh in these photos?

image image

Notes

  • Regarding JindraLacko's answer, I find it unbelievable that photographer put stickers in the center of their lenses to achieve this effect. I tried briefly obstructing the center of a lens to see what it would do, and the effect is quite different from the one seen in the sample images.
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The doughnut bokeh of catadioptric ("mirror") lenses is not caused by the rear mirror or long focal length, but by the obstruction (front mirror actually) on the front element.

You can achieve it with any lens if you put a round sticker on the front lens. Or better: on a clear (e. g. UV) filter in front of it.

With a little practice you could even achieve a heart or star shaped bokeh.

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  • \$\begingroup\$ The ring/donut shape bokeh is usually considered undesirable. Would someone try to create it intentionally, as you describe? \$\endgroup\$
    – xiota
    Commented Jun 30, 2018 at 12:26
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    \$\begingroup\$ Yup, quite often. Do a quick search for "bokeh rubinar" (a commonly used catadioptric lens) on Flickr to see what I mean... \$\endgroup\$
    – Jindra Lacko
    Commented Jun 30, 2018 at 13:27
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    \$\begingroup\$ @JindraLacko Rubinar is basically an English transliteration of the Russian word for "mirror lens." \$\endgroup\$
    – Michael C
    Commented Jun 30, 2018 at 21:01
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    \$\begingroup\$ @xiota "usually considered undesirable" is almost an opposite to "always undesirable" in art :) \$\endgroup\$
    – rackandboneman
    Commented Sep 25, 2018 at 11:37
  • \$\begingroup\$ @MichaelC not exactly. Rubinar is just a brand name of Soviet mirror lenses. USSR being USSR, there were no other brands, but this doesn't make it a generic Russian word, just like "Aeroflot" doesn't mean "an airline". \$\endgroup\$
    – IMil
    Commented Feb 17, 2020 at 4:51
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This bokeh effect is known as (soap) bubble bokeh. Along with a "glowing" look, this type of bokeh is seen in lenses that have over-corrected spherical aberration. It is associated with Cooke Triplet lenses, which have three elements in three groups. Myer-Optik Trioplan lenses, such as the 100mm F2.8, are particularly known for their bubble bokeh. 

Cooke Triplet

This image, taken with a Steinheil-Munchen Cassarit 50mm f/2.8, demonstrates both bubble bokeh and glow.

bubble bokeh and glow

Further reading:

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