Introduced by Tamron in 1980 * http://adaptall-2.com/lenses/20A.html My initial shots with this lens were very inconsistent - the ones that were good were very good but there seemed to be far too many shots that were mediocre. This lens uses an Adaptall-2 mount that is specific for the camera brand, i.e., Nikon, Canon, Pentax, etc. There seems to be a connection between the quality of the images and how the adaptall is mounted on the lens. My observation is that the lens should be set to the smallest aperture and that should coincide with the aperture indicator on the adaptall mount - that seems rather obvious but it is very easy to mount the adaptall and NOT have the numbers in sync.