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I had both lenses: 3.4 is a new world but very sensitive in strong light in the pictures.
The "SMALL" 4.0 with the common 39E filters is acceptable from 8.0 - if You can buy one 4/21 - take it and think about the limits...
The 3.4/21 is i.m.o. more "modern" in sharpness and contrast.
Patrick, Yes, the 21mm f/3.4 is superior to the older f/4 lens. I'va had the same 21mm f/3.4 since 1968 and have a 21mm f/2.8 for well over 20 years. I use the f/3.4 lens for black & white only. It vignettes and the effect looks dramatic in B&W. The 21mm f/2.8 I use basically for color. It has much less vignetting and is a half stop faster. While on an optical bench the 21 asph is better, in practical photography you can't see it. My main reason for getting a 21 asph is the 55mm filter size. Did you ever try to find a 60mm CC30M filter. Happy Snaps, Sal