Early version have a push-on mount, later a spring grip was used (subject of patent). A 'telephoto' was also produced.

The Proxar and Distar when introduced were intended as focal length modification lenses for the Tessar and Triotar. The Proxar was introduced around 1925, the Distar had been introduced some years earlier. With the introduction of miniature cameras such as the Rolleiflex and Contax the Proxar was used as a close-up lens. Early Proxars have brown mounts.

These are simple lenses placed in front of the camera lens, they can be negative - to increase the focal length or positive - to shorten the focal length.

They have been used in a number of ways:

• Focusing attachments

  A set of positive lenses was sometimes provided with box or detective cameras to give a focusing capability.

• Close-up lenses

  Typically used on cameras with single extension. These positive lenses allowed closer focusing. When introduced in the late 1890s they were often sold as copying, enlarging or portrait lenses.

• Focal modification

  Here the purpose was to alter the angle of view. Used on bellows cameras where the extension could be altered. Positive lenses gave a wider angle, negative lenses decreased the angle (wrongly called telephoto attachments).

• Film plane correction

  In colour processes where a screen plate is used the emulsion lies at the back of the sensitive plate and the film plane will be different to that shown on the focusing scale; using a low-power negative lens will bring the two into register. This was used in the Zeiss Ducar filter for use with Autochrome plates. The film plane register could also be different when changing from dark-slides to film packs, again a negative lens could be used to align the two. Goerz supplied such a lens for use with the film pack adapter of the Pocket Tenax.

With focusing and close-up lenses, with the primary lens set at infinity the focal length of the attachment gives the object distance.

The power is sometimes given in dioptres, this is the reciprocal of the focal length measured in metres.