Of course, cost is also a factor. But, unlike AF lenses, fully manual (incl. manual stop-down) lenses where cost is not an issue, you still can find plenty of blades. Case in point, my Hartblei/Carl Zeiss Superrotator 80mm f2.8 with tis 12-bladed aperture.
The main reason is the auto-aperture function, where you can meter and focus wide open, and then stop down to the shooting aperture automatically as the mirror flips up, before the shutter opens. Starting with some late Graflex reflex cameras, auto-aperture is a relatively modern phenomenon, and it's hard to make it work with 20 aperture blades, without adding an uncomfortable amount of shutter lag. My Zeiss 85/1.4 ZE has 9 blades, which is about as many as you'll find in a small SLR lens with auto-aperture function.
I do notice that rangefinder camera lenses tend to have more aperture blades, while SLR lenses tend to have fewer. And from a mechanical and physics point of view, it does make sense that it would be complicated to move 11 or more interlocking pieces of paper-thin metal in a fraction of a second.
Have you looked at the angle of the rounded more than 6 blade apertures? The curve and angle can introduce more sticking and error and shutter failure that need repair more often, I think it has to do with the mechanism and how long it will last without repair than anything, it doesn't cost them THAT much more for 8 blades then 5, it's that you introduce more components that need to be smaller to function and could easily break more often. I don't think it had anything to do with the aesthetic.
Mamiya: 7 II, RZ67 Pro II / Canon: 1V, AE-1, 5DmkII / Kodak: No 1 Pocket Autographic, No 1A Pocket Autographic | Sent w/ iPhone using Tapatalk
Ilex Paragon 6-1/2inch lens in a No. 3 Acme Synchro shutter has ten aperture blades, and my Olympus OM 100mm f2 Zuiko has nine aperture blades and it is an automatic aperture. Not so much difference. Cheap lenses have fewer aperture blades.
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