Luminous Landscape Forum
Site & Board Matters => About This Site => Topic started by: dchew on July 14, 2018, 09:15:46 am
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Thank you for reposting Harold's article. I have a common but eclectic mix of lenses, and was always curious why some diaphragms are so simple and others so complicated. I wondered why some, like the Zeiss 250 superachromat wouldn't have something more complicated than a simple 5-sided pentagon. The article gave some hint as to why. Here are some examples:
All my "large format" lenses are modern versions with Copal 0 shutters. 7 blades, but each side is slightly concave. Here is the Rodenstock 90hr-sw:
(http://www.davechewphotography.com/temp_images/diaphragm/copal0.jpg)
One of my favorites because of its small size is the Leica 90 f/2. 11 blades, again with a curve to each side but the curve is convex as opposed to the Copal 0 concave structure:
(http://www.davechewphotography.com/temp_images/diaphragm/90sum.jpg)
Here is the Leica 180R f/2.8 apo. This is the only lens I've seen with 8 blades; there are interesting hooks between f/2.8 and f/11. Beyond f/11 they are straight lines:
(http://www.davechewphotography.com/temp_images/diaphragm/180r.jpg)
The Zeiss/HB 250 superachromat is a simple straight 5-blade pentagon. Curious this one is so straight forward. Perhaps because of the design focus on reducing CA?
(http://www.davechewphotography.com/temp_images/diaphragm/250sa.jpg)
Of course the diaphragm on all of them disappears to reveal a sphere when wide open.
Dave
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The post-WWII W. German version of Zeiss' 50/1.5 Sonnar for Contax rangefinder cameras uses aperture blades that create a star shape (much like the Leica 90mm example in Dave's post above) when stopped down a bit. I think the intent was to let in additional oblique light rays in the f/2–5.6 range and so reduce focus shift. Doesn't work very well for that but it does soften the edges of "bokeh balls."
-Dave-