I have an older series Sigma AF 14mm f/3.5 lens -> http://www.bythom.com/14lens.htm I am wondering if anyone has had experience with this lens and using filters. Due to the extreme angle of view, it does not have any way of attaching a filter on the front ... and does not have any rear mounting. The only thing I can see is the lens cap attachment. It has a 72mm threaded opening. Since I am using this on an APS-C digital camera ... I am thinking that it may be possible to mount a very thin 72mm filter on this cap.