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RE: [OM] How many aperture blades on 50/2 macro?

Subject: RE: [OM] How many aperture blades on 50/2 macro?
From: Ken Norton <image66@xxxxxxx>
Date: Wed, 09 Feb 2000 12:24:58 -0600
Actually, the number of blades has little to do with resulting Bokeh, it's
the shape of the blades and optical formulas.  Lenses that have extreme
ranges in F-stops, like F2-32 need many blades to accomplish the task
without increasing the barrel size of the lens.  Rarely will you find a
lens that goes beyond F22 that doesn't have at least 8 blades.  The lowly
XA actually only has two blades shaped to mimick four.  I've actually seen
2-bladed lenses that had a nearly perfect round whole at all aperatures.
The blades were huge, though, and used something like a five point hinge
system.

The Zuiko 100/2.8 (my lens of legend) is an interesting bird as it only has
six blades and at 
aperatures less than F8 the blades have a serated characteristic to them.
Does wonders with bright lights (star effect), but the Bokeh is incredible.
 The blades are rounded, though and the opening at the smaller F-stops is
more round than many 8 and 9 bladed lenses.

You will find that most zooms have numerous blades and 9 blades is quite
common.  Unfortunately, if you notice, the blade edges are usually flat or
nearly flat.  So now, instead of a nice six-sided star effect you get this
completely unnatural 9-sided star effect that would never occur in nature.
(nine is not a naturally occuring number--1,2,3,5,8,13,21...).

I think it was Mike V. that just mentioned something said at the PMA about
lenses being designed for CCDs producing a more parallel light output vs
the angled output (greater angle of incidence the further away from the
center of image) of traditional lens designs.  Optical formulas are what
make or break Bokeh not the iris.  Even if you took the center out of a
mirror lens, for instance, you would still end up with lousy Bokeh, just
without the donuts.

Ken Norton


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