Factors that contribute to grainy bokeh

[onasj]

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Since there are multiple threads about this topic, here are some conclusions after testing with various lenses and conditions.  

The grainy bokeh phenomenon arises when microscopic imperfections in a lens element's surface—typically aspherical ones due to the way they are manufactured by press-fitting into a CNC-machined mold that is not perfectly smooth—alter the texture of bokeh from out-of-focus light sources.  Graininess is higher if:

1) There are more ASPH surfaces in the lens, because the graininess from each surface's imperfections will overlay and occasionally be additive, resulting in semi-random patterns as opposed to concentric onion rings;

2) The lens is sharper;

3) Bokeh is larger (that is, the lens's focusing distance deviates the most from the light source's actual distance and the aperture is as open as possible);

4) The out-of-focus light source is a close to a point source of light as possible (a tiny fiber optic or LED will result in much more grain than a car headlamp or a large streetlight bulb)

Bacuse the 35 APO-M 1) has four ASPH surfaces, 2) is currently the sharpest lens wide open in the M stable; and 3) can be set to focus closer than any other Leica M lens, it hits the trifecta and with point sources of light that are highly out of focus the graininess of the bokeh can be quite pronounced when viewed at high magnification or printed large.

Overall, this issue is a small downside to what is otherwise an optical masterpiece, and conditions have to be just right (wrong) for the effect to be sufficiently pronounced to distract, in my opinion.

 

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