In article <4bjkm2$dgd@peng.ping.at>, r.lehner@ping.at says... (..)
>When I look though it with the aperture closed the eight blades
>doesn't make a symmetrical figure - one is a bit to close, so the hole
>isn't "round".
>I took a look through my other lenses, and my other Tokina, a 17mm 3.5
>with six Blades doesn't make a symmetrical hole either.
>My two Nikkorrs - 50mm 1.4 and 135mm 2.8 have seven blades each and
>seen quite round, but not totaly. (..)
>Does anyone know how the the shape of the aperture concerns the image
>quality? I know that lenses perform generally best at a medium
>aperture and that it is impossible to get a perfectly round aperture.
>Isn't the symmetry of the aperture that important after all?

Ideally the aperture should be symmetrical (and the correct area) at
all apertures, but it rarely is. In practice, there is little problem,
unless the aperture is VERY assymetrical (which causes some difference
in the orientation and onset of diffraction effects with aperture
setting changes - probably subtle in effect, at worst [The cat's eye
is a good example of an optical system with a diaphram with extreme
asymmetry, and you don't hear cats complain about it, do you? {It may
serve to improve vertical resolution for the cat at the expense of
horizontal resolution at small stops while allowing for an extremely
wide aperture range - and the offset in diaphram angles between the
two eyes may also be helpful to the cat, but we use one lens at a time
on our cameras. ;-}]), or unusually incorrect in area (causing
inaccurate exposures, most likely at the smallest stops. [Small errors
in area usually go undetected, and even symmetrical diaphrams are
likely to be somewhat in error at small stops. And most lenses are not
effectively the marked aperture wide open {even though the aperture is
correct} because of design compromises {long lenses may be slow, and
short lenses may show illumination rolloff wide open}]).
Hope this isn't more than you ever wanted to know about apertures!
(There's more!)
Hope this helps.