On Fri, Mar 21, 2003 at 11:39:25AM -0600, Karen Nakamura wrote: > >I wasn't thinking of building one into the body, but a removable > >one that could be inserted between the body and lens, like TCs. > > That's very difficult for wider angle lenses. Look at the rear > elements of your wide angle lenses vs. your telephoto lenses. You'll > find in most cases at the widest setting, the rear element extends > considerably into the camera body. Even with modern retrofocus > designs, most rear elements of wide angle just barely clear the > reflex mirror.
True. However, at least to some extent that's due to the need to cover the entire frame, i.e., because of the relative sizes of the lens mount and the film or sensor size. That would be less of an issue in a converter specifically made for a smaller sensor. > TCs work because it's very easy to take the central portion of an > image and blow it up. Why would that be easier than the reverse, that is, taking the entire image and squeeze it to smaller size? Indeed, simply reversing a TC might do it (if it would physically fit without hitting the mirror, that is). > They also work because when you lengthen the > focal length, you give yourself more room to work. That is a valid point. Good TCs (like Canon's) extend into the lens, and there's no room in the body for similar design WACs. There are, however, 3rd party TCs that don't do it, and while quality suffers they're still acceptably good to many. Perhaps that restriction makes it impossible to come up with a design Canon would consider good enough. One can still hope a 3rd party known for cheap not-so-good TCs might jump in. > A wide angle converter after the lens unit will vignette Yes, of course it cannot recoup light that the lens has already lost, and indeed that is the main reason (as I understand it) why body-mounted wide angle converters cannot in general be made. But the situation is different when we have lenses that have been designed for larger frame size than the body actually has: concentrating the image that comes through the lens to a smaller area should be possible in principle. Or as you say: >-- you may > argue that with the 1.6x magnification factor of the consumer SLRs, > that's not an issue; but no one wants to engineer a lens adaptor > with an unsure history (my guess is that in 5 years, everyone will > be shooting full size imaging sensors). That is probably the most important reason. Such a converter would indeed necessarily be limited to specific sensor size and thus useful to a few bodies only, so they wouldn't sell enough to make it profitable. :-( > Anyway, I'd suggest getting a good lens theory book (like _History of > Photographic Lenses_) and looking through the lens construction > issues. You'll quickly find out why telephoto lens extenders are > easily designed (although difficult to design right) and why wide > angle converters have historically been filter mounted. Thank you for the reference. Am I not right, however, in that the main reason is the vignetting problem described above, and that it doesn't really apply to the situation at hand, where lens image circle far exceeds picture frame in the body? -- Tapani Tarvainen * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
