There is a drop in filter holder that takes 52mm filters designed to screw on to the front of lenses.

(For those unfamiliar - it works this way because the objective element of this lens is far larger than anyone makes filters for. 52mm is a very common diameter, so placing a drop-in mount further along in the focal path means you can use a wide range of filters with minimal cost and inconvenience, and to identical effect - except, of course, in the rare case where you have enough energy coming down the barrel to damage the iris.)