Optical filter which is coupled in front of the lens of a camera in order to achieve a certain effect on the picture. The filter can be a square glass attaches to the lens holder through an accessory called, but is more common in crystal form with a round ring-shaped frame metal or plastic screw. The objectives usually incorporate a thread for these filters. Obviousmind, the diameter of the thread must be the same on the filter and the lens. This is indicated by the symbol or the front lens and the edge of the filter and mm.