The Bellows Adapter M39 Thread to Leica M Lenses from NOVOFLEX allows a Leica M lens to be mounted to a NOVOFLEX Universal Bellows BALPRO, CASTBAL T/S or BAL-F. Each of these bellows requires an intermediate adapter (PROLEI, UNILEI, FLEXLEI) that converts the bellows standard to M39 Thread to accept this adapter. Although the lens will fit physically, automatic diaphragm (AE metering), or any other functions are not retained using this adapter. In this case you will need to meter in "stop-down mode" since the lens does not have the ability to have its aperture controlled by the camera body.