Quote Originally Posted by AgX View Post
The speed difference is negligable, so one could think Canon made a high-end version and a version with lower image quality that is cheaper but still bears that prestigious under-1 speed mark.

However your findings (confined to your two samples) show the opposite, the more expensive version is the minor. Seen that there is no practical speeed difference, that is puzzling. At first sight.

With SLR lenses we got the phenomenon that at the largest aperture of the high-speed lenses the image quality is reduced. But still one gains one stop in speed AND at smaller apertures the lenses yield better image quality than their counterpart of lesser price and speed.

In this case I could imagine that the image quality of the 1.7 lens would be better at smaller apertures than with 1.9 lens.
Just a guess...
With my Nikons, that was not my experience. I had two 50/1.4 and one 50/1.2, all late pre ai. I discovered that at f:2, the 50/2 Nikkor H was as good as or better than either of the faster lenses at the same (f:2) aperture. At f:4, still no edge to the faster lenses, and the 1.2 was a handful - a fair amount of flare even at smaller apertures due to all the area of all those big elements bouncing light around inside.
I now have two of the f:2-Hs a 66 and a 70-71, they're the only Nikon 50s I have.

But, the fact remains that an f:1.4/1.2 lens can sometimes get you a picture when a one stop slower lens won't.