Wednesday, January 23, 2002, 5:17:50 PM, Shel wrote: SB> Nine aperture blades is a good starting point. If you look ay many SB> older, quality lenses, you'll sometimes find it hard to count the blades SB> - there are sometimes a dozen or more. For example, the mid- to SB> late-fifties Leica Summarit (which I think you played with in Santa SB> Barbara), has 12 or 15 aperture blades, and the aperture is as close to SB> a true circle as I've ever seen.
SB> The question that comes to my mind is this: what's the advantage or SB> disadvantage of a greater number of blades, apart from the shape it SB> contributes to OOF highlights and the like? Some newer Leica lenses SB> have the most odd-shaped aperture opening, looking more like a six or SB> eight pointed star rather than a pentagon, octagon, circle, etc. SB> I don't like the OOF image on those lenses - they are star shaped. SB> Steve Larson wrote: manual aperture, pre-set and rangefinder lenses can have as many aperture blades as they want. My Carl Zeiss Jena lenses from the pre-set period have about 12-20 blades maybe. Let me tell you the 180/2.8 with its about 18 blades has perfectly circular bokeh at all apertures... but with auto-aperture and semi-auto-aperture the speed with which the aperture closes and opens needs much less blades. That's why the 9 blades are maximum, and 5-6 blades are usual. There's less stress on the mechanism. With 18 blades, it wouldn't close as quickly as required for 5 fps motordrive or 1/2000 time. The Leica highlights are really strange, often a many-pointed star surrounded by bright ring, in sun-water reflections and highlights. I guess at least some of it is attributed to bad coating (did I read on Leica list that Father Leitz had the attitude that multicoating is not really necessary if lens design is good?). Any other ideas? Good light, Frantisek Vlcek - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
