Extension tube

Lens tube extensions are available for many lens mounts of modern camera systems. The simplest are the macro rings. The extension can be mounted on the camera's lens mount, and then the lens on the extension's own lens mount. This construction enables the photographer to shorten the smallest distance of the image subject to the front element of the lens.

A sketchy profile of a camera without and with lens tube extension, and the optical path of the light going from the image subject to the focal plane. Remarkable: the reduced depth of field in the macro mode. image by Uwe Kulick (Image rights)

Lens tube extensions might have facilities to transmit the aperture release shift from the camera.

Exa's reverse aperture transmission concept: The levers on the Exa's macro rings elongate the shutter release button so that it still can be triggered by pressing the lens's aperture release lever image by Uwe Kulick (Image rights)