Your friend could always read the treatise by Dr. Nasse on bokeh as
well, if all else fails. ;-)
www.smt.zeiss.com/.../CLN_35_Bokeh.../CLN35_Bokeh_en.pdfCachedSimilar
To extract from his article about all the factors influencing the
nature of bokeh:
"All the parameters listed here influence the phenomena outside the
focal plane:
•
Picture format--(i.e.size of sensor or film--added by MG)
•
Focal length
•
f-number
•
The camera-to-subject distance
•
Distance to the background or the foreground
•
Shapes and patterns of the subject
•
Aperture iris shape
•
Aberrations of the lens
•
Speed of the lens
•
Foreground/background brightness
•
Colour"
One number to rank it just is plain misleading. The zuiko 50/1.4 was
badly graded but in some conditions it is fine. One thing for sure is
it is a zero sum game in that if the lens is otimized for nice
background bokeh the foreground will suffer. Highly corrected lenses
for spherical aberration will also in general have harsh bokeh--witness
the OMZ 50/3.5 macro. The degree of blur with a distant background is a
function of the physical size of the aperture---so longer FL and larger
formats have an advantage.
One cool way to learn about bokeh would be to obtain a Helios 44-2 58mm
f/2 Lens--used to be 8 bucks on yabe from russia or Ukraine--mebbe more
now. It is buttery smooth wide open (corners are mush then) but stop
down a bit with subject (IIRC) at about 8 ft with distant background
and one gets wicked cool swirl bokeh. It is a devilishly finicky Diva
to get a compelling image, but if all is correct, it is very nice
indeed.
Oh, found one with a quick search:
http://a.disquscdn.com/uploads/mediaembed/images/1225/1670/original.jpg?w=600&;
How's that for bokeh?
I think it was the style with Petzval lenses lenses in LF and the
recent expensive incarnations available ---just like buttery smooth may
be the style to some degree now in some quarters. I think it is not due
to any optical characteristics per se, but just internal vignetting.
One can mimic this with just about any lens by masking the front
element to an aperture just larger than the entrance pupil (at the
price of substantial vignetting).
Dr. Bokeh ?, Mike
--
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