I've only just noticed this thread. I used to use a Nikon FE for IR work. The lens I used had a 52mm thread. It was the 28mm E series. I found an Optech lens cap which screwed in and was hinged. I don't know if they are still made. I drilled out the soft plastic cap and screwed a Hoya R72 into it. There is no thread, but the filter cut its own thread into the plastic. It's easy if you can find the Optech cap. It doesn't work with my 24mm, but is ok with 28mm and above. Alex