What size infrared filter should i use for my 50mm lens?

Answer 1

Remove your lens cap and see what size the front of your lens is. (It tells on the back of the cap.) Most 50mm lenses take 52mm filters, but some take 49mm.

Answer 2

Actually, the size isn't usually shown on the rear of the lens cap, but around the edge of the lens. It will be an 'o' with a slash through it followed by a number. That number is the size of filter you need.