If I may shed a little light on the Russar 20 mm, this has a very exeptional form of the exit pupil. This reduces the need for a ND filter, as Marco Cavina has explained in depth:
lichtval Russar kopie.jpg
As you see, the pupil remains the same form even wide off from the center. Marco Cavina technically says this is an exceptional lens in that it follows a different optical paradigm.
I was curious if the 15/16mm Hologon would follow this this 'rule' of Cos^3-Theta too, but I am afraid it is not, rereading Cavina's work; he would have mentioned it. IMHO, this makes this Russar lens more fit for digital unless you have the Hologon ND filter.
I think my old Russar 20mm copy - disregarding the fact that it is 19,7mm - is no match for the 16mm Hologon in coating and probably in sharpness, but it performs beyond expectation on the MM, with very clear details into the corners, a feat not easily met by other wide angles.
It gives a beautiful "WA-bokeh". And dark areas on my Monochrome-I can be reaised exceptionally.