Ha! I've had my Zenitar 16/2.8 for 3 years now and I never noticed the filter before! I just tested it -- removed the filter, tried to focus, and found that its far-focus dropped from infinity to about 1m. Yes, the filter is necessary! For anything except closeups, anyway.
Originally posted by elliott The clear filter says M26,5x0,5 and C on the outside ring, it should be visible when looking at the mount of the lens.
That's just what mine says.
Originally posted by elliott The colored ones are useless on digital, but you need the clear one, otherwise the Zenitar will not focus properly. The Zenitar filters are special, they are part of the lens.
Yes, the colored filters are worthless for most digital photography, EXCEPT:
* a RED filter gives slightly greater dynamic range when shooting B&W
* a YELLOW filter does wild things to glaring colored neon lights at night
I may search for those filters, just so I will have them, just in case...