I have an insane dream to do some wildlife infrared work with my medium format camera. My big MF lens is a Pentacon 500mm f/5.6 mounted on a Pentax 645. The front threads of this bwahemoth measure at 118mm which is an odd size (this is a German lens over 20 years old). Does anybody know wher to find filters this size? Is there an alternative to a normal front-mount filter? And I am not interested in a custom made filter because it would probably cost more than the camera/lens did. I have Maco 820c and Kodak 2424 film available.

I have an insane dream to do some wildlife infrared work with my medium format camera. My big MF lens is a Pentacon 500mm f/5.6 mounted on a Pentax 645. The front threads of this bwahemoth measure at 118mm which is an odd size (this is a German lens over 20 years old). Does anybody know wher to find filters this size? Is there an alternative to a normal front-mount filter? And I am not interested in a custom made filter because it would probably cost more than the camera/lens did. I have Maco 820c and Kodak 2424 film available.

You can get gel IR filters from Kodak in some pretty large sheets. I'm pretty sure they make holders big enough, at which point you'd only need a thread adapter. Try B&H and Calumet.