Too both I-61 and I-26 are easier to use with a filter (try to change the I-50 aperture with a filter in place... ).
The Jupiter-12 has the same clever design. I haven't used mine in a while, but if I remember correctly, it's possible on that lens to apply a little lateral pressure to the filter and get the aperture to move---but still pretty tricky to see *what* aperture you're selecting! Did the original Biogon have the same feature, or was it a Soviet innovation?
San Diego, CA, USA
The lady of the house has to be a pretty swell sort of person to put up with the annoyance of a photographer.
-The Little Technical Library, _Developing, Printing, And Enlarging_