A macro filter, also known as a close-up lens or diopter, is an optical element that screws onto the front of a camera lens. It functions similarly to a magnifying glass – it shortens the minimum focusing distance of the lens. This allows the photographer to get closer to the subject than their lens would normally allow, while keeping it sharp. The result is a significant magnification of small objects and details in the photograph, which is the principle of macro photography, i.e., the technique of close-up shooting. The strength of these filters is given in diopters, for example +1, +2, or +4, where a higher number means greater magnification. They are often sold in sets with various dioptric values, which can be combined to achieve even greater magnification. They represent an affordable alternative to specialized and more expensive macro lenses.