Lenses: Form of the Iris

Circular diaphragms are actually only possible with approx. 10 blades and more. If there are only 6 or 8 blades, the diaphragm might be round for the first few stops down, but after this probably still being rounded, but not circular.

Ken Rockwell states this for the EF 16-35mm F2.8L II for example, where the aperture is perfectly round until F4 and in fact polygonal from F8 upwards. Same thing for the EF-S 18-55mm F3.5-5.6 II and EF-S 18-55mm IS (both by 2 stops), which Canon markets as featuring a circular aperture. Even EF 50mm F1.2L with its 8 aperture blades is only truly circular up to F2.8, polygonal from F4 upwards, similar for EF 85mm F1.2L II (round to F2 only).

Anyway, where as many as possible aperture blades with an as round as possible iris opening is often rumored to be the basic reason for great bokeh, there are several comparisons of lenses available that will prove you wrong. A multitude of aperture blades can indeed manage to revive some bokeh, but in a few cases even lenses with only 6 blades in a basic polygonal shape did beat other lenses with much more blades in a rounded shape, simply because bokeh is a matter of glass in the first hand. Source to come, I think it was in Bokeh Battle on 16:9.

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