I am about to buy a circular polarising filter and 2 neutral density filters (ND4 & ND8) and I see that there is a choice of either a push-on type or a threaded screw-on type. What do people use most and has anyone had any experiences of the push-on type slipping off? I imagine the risk is greatest when facing the camera directly down. Do you remove the standard UV filter that is in place when installing your filters? Thank you, Pete.