[Leica] Bokeh and quantification

[frank.dernie at btinternet.com (FRANK DERNIE)]

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.....
Frank
--- Marty Deveney <freakscene@weirdness.com> wrote:

> I have just finished reading a thesis in which bokeh
> is
> quantified precisely.  The system is based on a
> series of identical
> digital black and white images, taken with three
> lenses (they happen to
> be large format because of the scanning back that
> she used but could be
> any lenses) at various apertures and focal distances
> - the pixel
> distribution was then measured around key in- and
> out-of-focus areas (to
> capture a representative range of parts and
> characteristics of the
> image).  This could be done with a film image by
> fine-scale densitometry,
> so the system is not just applicable to digital
> images.  She then used a
> dissimilarity index to compare the points.  This
> resulted in a graph that
> quantifies the changes in pixel distribution from
> near to far for a given
> aperture.  That part of the method quantifies _what_
> the bokeh looks like
> at that aperture.  The different apertures are then
> compared using a
> scaling analysis that provides a three-dimensional
> representation  of
> dissimilarity - a virtual, mathematical
> representation of the bokeh, if
> you like.  If you superimpose these plots from the
> three different
> lenses, you have an absolute representation of the
> differences in bokeh. 
> So it can be done, even if it took a year to do it
> for three lenses.  The
> method Canon uses is simpler and less precise than
> this, but does also
> capture it effectively.
> 
> So it can be done.  The student did mention in her
> discussion, however,
> that while the method was sound and proved bokeh
> could be measured,
> that bokeh was probably best judged visually by the
> photographer . . .
> 
> -- 
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