I would like to experiment with filters on my scope. It is a Leupold FX-3 25X. I tried a 43, too small and a 46, too large. Will the threads match up with a standard camera lens filter? What size?

Leupold makes filters for their scopes. If you have an older scope, you need an adapter ring because they changed the threads on the objective lens. The 25x that you have mentioned is the new thread.

I have tried all three colors they have and I can't see that they make any difference. If you make it to a match at TRC you are welcome to give them a try.

Maybe a UV filter to make things sharper?

Maybe, but the size is the question.

I got an answer back from Leupold. I think that it is a non-standard thread.

"The thread pitch that you’ll want to look for is 1.743-48."

It's about 44.27 mm.