It's ongoing but accelerates in the 90's and then speeds up in the past 10/12 years. These changes mean both types of wide angle lenses improve, the Cosina made WA's for range finder cameras are outstanding optically.
Originally Posted by David Lyga
I use a 17mm Tamron SP (my second) although I'd buy a 21mm Leitz lens for my M3 if I needed a lens like this for commercial use (with film). I much prefer the 21mm Leitz lenses but would buy a Cosina eqivalent if the budget was too tight however I can't justift it these days as I rarely use 35mm.