Lens adapter

[2] This type of adapter is designed to reduce focal length, increase lens speed, and in some instances improve MTF performance.

Flange focal distance is the key specification in determining whether or not a particular type of lens can be adapted to any given camera.

Generally speaking, cameras with a shorter flange focal distance can be adapted to a greater diversity of lenses.

This introduces the typical side effects of a teleconverter, namely decreasing the amount of light that reaches the sensor, and adding a crop factor to the lens.

Finally, some adapters may include electronic or mechanical provisions in order to control the lens aperture and autofocus, to record the correct Exif data, or to report focus confirmation.

Two passive adapters to mount lenses to Sony E-mount cameras – for Exakta (left) and for Minolta SR/MD lenses (right).
This lens adapter is a passive adapter designed for mounting a Nikon F-mount lens to a Micro Four Thirds camera.
Active lens adapter: Canon EF to Sony E.