The 21cm heliar I got, has a 0,75x57:ish thread on the inside of the front element, so if I've understood correctly one has to use push/slip-on type of filters with this lens (60mm because its technika branded)? Why is there a thread on the inside of the front element?
Problem is I'm not finding suitable filters or, even better, step-up rings. (like 60mm push-on --->67mm would be ideal) Does something like this exist?
Any pointers?
Thanks,
Anders
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