I have to agree with Michael R

Quote Originally Posted by Michael R 1974 View Post
Being aware of and/or interested in how things work does not make someone any less a photographer, nor does it make someone a "wanker" of any kind. The wankers post to technical threads to remind everyone photography is about pictures. Brilliant.
The OP asked a valid question and there's no doubting that the best extreme WA lens sdesigns have been made fo rangefinder cameras and aren't made for SLR use (unless mirror lock is used).

That doesn't mean that there aren't good extreme wide angles for SLR use, it's just that they behave slightly differently. If they weren't there would be no need for Hasselblad to have made the SWC cameras, Nikon their special 2.1cm f/4-16 Nikkor-O etc.

The OP's asking about those differences and unless you've used both types of designs it is hard to appreciate the benefits of the purer designs of wide angle lenses for rangefinder cameras, or in the case of the SWC and Nikon lens where no mirror is used.