It is a secondary lens used to enable macro photography without requiring a specialized primary lens.

A close-up filter is a lens that attaches onto the end of a camera lens via a screw thread. The purpose of the filter is to decrease the minimum distance that a lens requires to focus. For example most telephoto lenses need the subject to be at least a metre and a half away before they can focus. By attaching a close-up filter to the lens you can reduce this to maybe 0.75 metres.

Close-up filters are measured in Dioptre, with +2 being weak ait is a simple secondary lens used to enable macro photography without requiring a specialized primary lens.nd +10 being strong. A dioptre is a measure of lens power.

Filters can be "stacked" together (one filter screwed onto the lens and another screwed into the first filter). When stacking filters always make sure that the strongest filter is closest to the lens.