Extension tube

[2] Lenses classically focus closer than infinity by moving all optical elements further from the film or sensor; an extension tube simply enables this movement.

Due to their function, there are other effects: decrease of light; shallower depth of field; and loss of ability to focus at infinity.

The precise reduction of the amount of light reaching the imaging plane can be calculated using the inverse square law formula.

More expensive extension tubes contain electrical contacts allowing the user to use autofocus and electronically control the aperture of the attached lens.

Extension tubes are sometimes confused with teleconverters, an optical component (i.e., containing lenses) designed to increase effective focal length.

A set of extension tubes with a pen illustrating the lack of internal lenses
Nikon F-mount extension tube
Vivitar zoom teleconverter, which acts like a zoom extension tube