Re: [Leica] no. of blades

[Alfred Breull <puma@hannover.sgh-net.de>]

The more blades, the more round - no, it depends on the shape, the 
position of the blades, and the opening (f-stop).

The Summar has 12 blades (2 times 6), which form  a hexagon between 
f 2.2 and 12.5; the Summitar (10) shows an almost perfect circle at
f 2.8 (or short before 3.2) and almost a circle before or after. The
rigid chrome Summicron (10) shows the circle at f 16, both my coll.
Summicrons (10) between f 2 and 2.8, and the Summarit (15) or 73 mm
Hektor (15) at all f-stops, with the circle being "more perfect" in
the Summarit. Besides, my post-war 1.5/50 Zeiss Sonnar (11 blades) 
shows the almost perfect circle between f 1.5 and 2. The 10 blades
of both the Summitar or Summicron (coll, rig chrome) show a more
perfect circle than the Sonnar's 11 blades. 

So, when you take a high contrast picture (sun or it's reflections 
shining into the lens) at i.e. f 16, even the coll or rigid chrome 
Summicron show different results, because the rigid has the iris 
circle at f 16 and the collapsibles between  f 2 and 2.8. Of course 
the shape of the "rays" depends on the space between the single 
blades also ... :)

Alf 


At 21:11 17.12.1997 -0500, Dan Post wrote:
>Interesting- I had heard that the greater the number, the rounder the
>aperture and hence somewhat better image formation ( diffraction?).