>So why do some people complain about those cute little doughnuts you get
>with mirror lenses? Just personal taste, I guess.
mirror lenses effectively have a diaphragm that is like a donut, ie
it is an annulus, not a disk. This creates highly non-smooth bokeh,
the donuts of a mirror lens.
the number of blades in a diaphragm matters for bokeh, but optical design
is more important. there are design tradeoffs when designing a lens,
and designing for good bokeh constrains the degrees of freedom you have
for designing for other things (sharpness, contrast etc.). That's why
soft focus lenses exist. You can soften a sharp lens with a
softener/diffuser, but if you are always goign to use the lens
this way, the optical designer can optimize the design for
contrast and bokeh at the expense of sharpness in the first place.
there is a major tension between how well corrected a lens is wide
open and bokeh also. some aberrations, notable spherical aberration,
contribute to smooth bokeh. this aberration goes away as a lens
is stopped down. Nikon designs their lenses for maximal wide open
sharpness at the expense of bokeh. This is why so many Nikon lenses have
the notorious double-line bokeh that some people don't like. They
aren't inferior because of it, they just are optimized for something
else.
note that when you shoot a lens wide open, the diaphragm is not
part of the picture, and you get as much variability of bokeh from
lens to lens when shooting wide open as when shooting stopped down,
except that as you stop down, bokeh becomes less and less important.
Joseph
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