In article <54fh5j$1dj@elektron.et.tudelft.nl>, jaw@elmat11.dimes.tudelft.nl says...

>Last week I had the opportunity to shoot a few slides with a Cosina
>3.8/20mm lense (thanks to the kind salesman in the shop). I'm acually
>considering buying a extra-wide angle for my Minolta X-700, and this is
>the best available and not-terribly-expensive 20mm lense I've found.
>However, I'm still not convinced of its value, because it has severe
>light fall-off in the edges of the picture (is this called vignetting?)
>when used wide open or at f5.6. Only from f11 onward acceptable pictures
>can be made.
>
>So, the question is: is this common for 20mm lenses or is
>this Cosina lense just very bad quality? And do you, experts out there,
>think a cheap lense with its obvious shortcomings is worth its money or
>not (meaning: don't use it at wider open than f11)? Any opinions,
>comments or advise....

If the lens is cheap, and produces an excellent image to the corners
(with even illumination - "vignette" is a sharp corner cut-off, as
caused by a too-small shade, or too thick filter rim) by f11, OK
(who cares with a 20mm what aperture was used to make the picture,
so long as it is sharp - and super wides can be successfully
hand-held at very slow shutter speeds...). But if the image is not
excellent at at least one or two apertures, there is little point
in owning the lens (except for entertainment value....;-). BTW, for
comparison, most samples of the Nikkor 20mm f2.8 are very sharp
to the corners by f5.6 (I avoid using it wider, if possible, since
the corners are somewhat soft wider than f5.6), and all are very even
in illumination (it is an excellent 20mm lens). You might look for
a used Minolta 20mm, also, which would probably be better than the
Cosina.
Hope This Helps