Dan, I-11M series is apparently a Soviet copy of corresponding Apo-Tessars, but a very well-made one, and with a slot for different waterhouse stops (for different halftoning screen patterns). Is RF-5 really apo corrected, I just don't know - but it should be still a fine lens, and what difference do you see with Apo-Nikkor - a field curvature, distortion, contrast issues, resolution power? They're different in optical design, so they have a full right to behave quite differently But RF as well as I-11M were used for the most precise work here, so they have to be good