The SP AF PRO-series teleconverters, with a newly designed optical construction (4-group/5-element for 1.4X and 4-group/7-element for 2X), are designed to be used with lenses that are F/2.8 and of focal lengths of 90mm or greater. This would make the use of the SP teleconverters ideal for the Tamron SP90mm F/2.8 macro, 70-210mm F/2.8 and 300mm F/2.8. Relative illumination is significantly improved resulting in better overall contrast and resolution across the entire image field. In addition, chromatic aberration and astigmatism are better compensated for by virtue of the use of higher-grade glass materials and more sophisticated design techniques than used in universal-type teleconverters. 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 comprise image quality.The 2X doubles the focal length of the master lens and makes the lens two f-stops slower. In addition to increasing focal length, this teleconverter allows greater magnification in macro since the Minimum Object Distance (MOD) of the master lens does not change when used with the teleconverters. AF will be retained with the 2X teleconverter if the master lens is F/4.0 or faster.