The Tamron SP AF Pro teleconverter features improved optical construction, and is designed to be used with lenses that are f/2.8 and of focal lengths of 90mm or greater.
The 1.4x teleconverter (also known as a tele-extender) will multiply the focal length of the existing lens by 1.4x, and will cut back the amount of light by one f/stop. For example, using a 1.4x teleconverter with a 300mm lens will effectively increase the focal length to 420mm. A lens set to f/2.8 will now be f/4.
This teleconverter is not recommended for lenses wider than 90mm, e.g. 50mm, 28-105mm, etc. However, full AF operation is still only possible with lenses with open aperture of f/2.8 or brighter. Please be aware that AF will work properly only if there is enough light and contrast on the subject to activate the camera's AF sensors. Manual focusing is recommended when using lenses with smaller open f-stop value than those given above.
Note: When used with lenses with smaller maximum apertures (f/4 and f/5.6, for example) there will be little difference in image quality as compared with the standard Tamron teleconverters. When used with lenses wider than 90mm the SP teleconverters can actually compromise image quality.