Tamron SP 2X Tele-Converter Model 01F: This 2X converter, compatible with most Adaptall-2 mount lenses, provides very good optical performance. The 6 element, 5 group optical design is housed in a moderately long barrel which is about twice the length of conventional 4 element 2X converters. The flat field optical design minimizes off-axis aberrations; spherical and chromatic aberrations are also extraordinarily well controlled. Tamron's literature states that this converter is designed for lenses with a focal length of 90mm and longer. Nevertheless this converter performs well with all Tamron zoom lenses which have a stationary master group (the rearmost group of lens elements). This converter can also be used with many Tamron wide to tele zoom lenses for doubling macro magnification, and usually results in better off-axis macro performance.
Note that Tamron also produced the very rare SP 2X Tele-Converter Model 200F. The 200F converter was designed to be used with Tamron's SP 300mm F/2.8, SP 400mm F/4, SP 200-500 F/5.6, and the SP 350mm F/5.6 and SP 500mm F/8 mirror lenses. The 200F converter can not be used most other Tamron lenses since the converter's front element will come in contact with the lens's rearmost element when you try to attach the converter.