Diopters and Split Diopters are simple single or two-element lenses used to assist in close-up and macro photography. They provide some number of positive optical diopters, which magnify the subject and allow objects very close to the lens to be brought into focus. They are sometimes sold singly, and sometimes sold in kits of +1, +2, and +4 diopters, which allows them to be combined to produce a range from +1 to +7.
A split diopter is a diopter in which only half of the camera’s lens area is covered by the filter. A round split diopter has a usual filter ring, but is filled with only a semicircle of glass (or plastic). This allows the photographer to photograph an object which is very close against a background much further away, effectively extending depth of field. Careful composition is required to make effective use of this device.