In photography

Photography

Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...

, a close-up filter, close-up lens or macro filter is a simple secondary lens

Secondary lens

In photography, a secondary lens is a lens designed to be used in conjunction with another lens, called the primary lens.A secondary lens may be designed to be used either in front of the primary lens, between it and the subject, or behind the primary lens, between it and the film.Secondary lenses...

 used to enable macro photography

Macro photography

Macrophotography is close-up photography, usually of very small subjects. Classically a macrophotograph is one in which the size of the subject on the negative is greater than life size. However in modern use it refers to a finished photograph of a subject at greater than life size...

 without requiring a specialised primary lens. They work identically to reading glasses, allowing any primary lens to focus more closely.

Close-up lenses typically mount on the filter thread of the primary lens, and are manufactured and sold by suppliers of photographic filter

Photographic filter

In photography and videography, a filter is a camera accessory consisting of an optical filter that can be inserted in the optical path. The filter can be a square or oblong shape mounted in a holder accessory, or, more commonly, a glass or plastic disk with a metal or plastic ring frame, which...

s. Some manufacturers refer to their close-up lenses as diopters, after the unit of measurement of their optical power.

While some single-element close-up lenses produce images with severe aberrations, there are also high-quality close-up lenses composed as achromatic doublets

Doublet (lens)

In optics, a doublet is a type of lens made up of two simple lenses paired together. Such an arrangement allows more optical surfaces, thicknesses, and formulations, especially as the space between lenses may be considered an "element." With additional degrees of freedom, optical designers have...

 which are capable of producing excellent images, with fairly low loss of sharpness.

Close-up lenses are usually specified by their optical power, the reciprocal of the focal length in meters. Several close-up lenses may be used in combination; the optical power of the combination is the sum of the optical powers of the component lenses; a set of lenses of +1, +2, and +4 diopter

Dioptre

A dioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres . It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at metre...

s can be combined to provide a range from +1 to +7 in steps of 1.
A split diopter has just a semicircular half of a close-up lens in a normal filter holder. It can be used to photograph a close object and a much more distant background, with everything in sharp focus; with any non-split lens the depth of field would be far too shallow.

External Links

• DIY Close Up Filter How to make a Close Up Filter for under one dollar