i've been seeing much of these. nikon 5t, 6t, canon 500d. how effective r these and wat r the side effects? do they darken the lens like polarizers do? lets say i hav a 85/1.8 and put on a close up filter, how would tis compare with a tamron 90/2.8? wat is meant by the diopter power? my tokina 28-80 focuses to 50cm, i put on a 6t, how close can i focus now?