Archived posting to the Leica Users Group, 2006/05/19

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Subject: [Leica] Bokeh and quantification
From: freakscene at weirdness.com (Marty Deveney)
Date: Fri May 19 04:26:30 2006

My interest has been, not that boke, or the character of the OOF areas
can be measured, but that the characteristic of the lens design which has
the effect could be measured. Many just mention a circular aperture but
since it is the character wide open which is of most interest to me
clearly the shape of the aperture has no
influence. Canon state in their Lens Work publication, without
explanation or discussion, that if the saggital and radial MTF curves run
together the boke will be pleasing. This completely agrees with my
experience in the case of the lenses I have been able to find MTF curves
for so I am prepared to believe it, but I'd like to know why. I was BTW
one of those who sold my CV 50mm f1.5 Nokton because of the boke being
awful. My favourite 50 is my pre asph Summilux, though I have never seen
never mind used the new one.....

This is relatively simple.  The differences between tangential and
saggital MTF curves indicate how well off-axis aberrations are corrected,
including astigmatism and field curvature; properties that affect bokeh. 
Bad bokeh is typically associated with these off-axis aberrations, which
is why a significant difference in tangential and saggital MTF
is indicative of the quality of bokeh.

Marty

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Replies: Reply from walt at waltjohnson.com (Walt Johnson) ([Leica] Bokeh and quantification)