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Re: [OM] bokeh

Subject: Re: [OM] bokeh
From: Joe Gwinn <joegwinn@xxxxxxxxx>
Date: Thu, 2 Jan 2003 17:35:00 -0500
At 8:24 PM +0000 1/2/03, olympus-digest wrote:
>Date: Thu, 2 Jan 2003 13:04:29 -0600
>From: "gries" <gries@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
>Subject: [OM] bokeh
>
>Is there a good test to determine the quality of a lens' bokeh?  Gary's
>tests are quite helpful in determining sharp lenses, but what about the
>bokeh?
>
>I have been immensely satisfied with the bokeh from the 85/2, but less so
>with the 28/2.  is there any source that describes  the different lenses in
>these terms?  What are the terms to describe bokeh (Double-line, etc.)?

I think that bokeh probably has a fairly simple technical explanation: that if 
the bokeh is good out-of-focus images are still smooth, and we don't get any 
odd artifacts like doubling of lines.

If my theory is true, the test is easy: just run a resolution test with the 
lens at varying degrees of defocus, and observe that the three-bar test 
patterns smoothly verge to gray as the lines per millimeter increases.  The 
more defocus, the lower the lines per millimeter at which the patterns will 
blur to gray.  Look especially for patterns where the number of bars in the 
pattern seems to double at some resolution.  This is an artifact; the lens 
resolution didn't suddenly double.  Sometimes you see this even with lenses in 
perfect focus, but good lenses don't do this one would expect

If one sees doubling of edges in defocused images, one will see prominant 
ripples in the corresponding modulation transfer function, rather than the 
traditional monotonic decreasing curve.

Joe Gwinn.



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