Reasons why they aren't built into the body:
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.
TCs work because it's very easy to take the central portion of an image and blow it up. They also work because when you lengthen the focal length, you give yourself more room to work. A wide angle converter after the lens unit will vignette -- 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).
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.
KN
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