[Leica] Star Diffraction Effect & Bokeh Solved!

[Robert Rose <RJR@usip.com>]

I thought that title would attract attention :-)

Seriously, for those of you who have followed the short thread on the star diffraction patterns that appear even at wide open aperture, let me suggest that I have found the road to a solution, and it suggests a qualitative approach to "bokeh" as well.

The Problem:
Several of us have noticed that in-focus point sources of light have star diffraction patterns using Leica lenses, much like a built-in star filter.  This can occur even wide open.  The effect can be seen on street lamps, Christmas tree lights, candles, etc.  

Why does this happen, and what are the variables which produce this effect?

Hypothesis:
The presence of the effect is primarily a function of the shape of the aperture opening formed by the diaphragm blades.  The number  of spikes will be a function of the number of blades, generally double the number of blades.  In the limit case of a completely circular aperture, the spikes will merge into a pattern of concentric circles.

The strength of the effect is a function of the quality of the lens and the f stop.  The higher the quality of the lens and the smaller the aperture, the more pronounced will be the effect.

As the point source moves out of the plane of focus the effect will become less pronounced, but the shape of the out of focus image will still be influenced by the shape of the aperture.

Methodology
To test the hypothesis I exposed a roll of Fuji Super G 400 in different three cameras (rewinding and reloading).  This eliminated the problem of different emulsion and processing.  I used a street lamp as an image, exposure 1 sec @ f/4, on a tripod.  (with appropriate changes in speed for other apertures.)  The tested lenses were:
35mm Summicron-M ASPH at f/2,4, 8
50mm Summicron DR at f 2, 4, 8
90mm Elmarit-M at f/2.8, 4, 8
Vario-Elmar-R 35-70 f/4, at f/4, 8
Olympus OM lenses:
       24mm f/2.8, 4
       35mm f/2.8, 4
       50mm f/1.8, 4
     100mm f/2.8, 4
        
I scanned the images into Photoshop, and they will be uploaded for viewing this weekend.

Conclusions
When the images are uploaded you can look and form your own conclusions, but it will be apparent that if the aperture is circular, there is no star pattern.  The "better" lenses had less ghosting and flare, higher contrast, and more pronounced patterns.

One very interesting result became obvious.  The Vario-Elmar-R exhibits the patten "wide open" at f/4.  Looking at the aperture, however, shows that it is really closed somewhat at that setting, and I suspect that the lens may have been designed as, say f/3.5 and then "closed" down because of performance.  Just a guess, but it means that you cannot eliminate the star pattern with this lens.

The aperture of the 35 Summicron ASPH never quite opens all the way, and therefore even wide open some effect can be seen.

The shape of the aperture changes on most of these lenses, so each f stop exhibits a unique pattern!

The king of circular aperture is the 135 Hektor, which I did not test yet.

If you have any thoughts, or you own results, let me know and I will add it to the research.

Bob Rose