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.