Willem,
> PS: no reaction on my earlier posted idea of a 0.7x converter
> *behind* the lens, to compensate the smal chip size?
> Why can't that work?
I thought about this question for a while, and the answer
came naturally while I was investigating image circles of
large format lenses.
A teleconverter is basically a device that crops part of the
image, and then "magnifies" it to full-frame.
Now each lens has an image circle with a certain size. The
image circle in large format lenses determines what size
of (sheet) film can be used so that every part of the film
still gets light during the exposure. If a larger format
than recommended is used, only part of the film will receive
light during exposure. (Kind of, but not quite like a
circular fish-eye.)
Small-format (i.e. 135 type) lenses are designed to cover
the full frame of the 35mm film, In a few cases, the image
circle of a 135 lens is large (or nearly large) enough to
cover 6x6 medium format.
Your proposed wideangle-converter will have to do the
opposite of what a teleconverter does - it has to extend
the image "frame", and resize that image down to 135
format. (Optically, this shouldn't be a real challenge,
it's kind of like mounting a TC in reverse.)
Now, if we extend the image "frame", for a lot of 135
format lenses we would have to go beyond the size of the
image circle, causing heavy vignetting in the corners, or
even a circular image on film.
Nobody would keep us from using MF lenses, right? Of
course not, but it really doesn't make much sense to go
from e.g. a 30mm MF lens to a 21mm 135 lens using our
hypothetical 0.7x converter. It is more difficult to design
wide angle lenses in MF than in 135 format - at the same
focal length. Easier and cheaper (less glass) to go with
the 21mm 135 lens.
Lars
--
Lars Michael [EMAIL PROTECTED]
87GT http://www.larsmichael.com/
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