Sigma APO Tele Converter 1.4x and 2x are dedicated Tele Converter lenses, which should be used with only listed at: http://www.sigmaphoto.com/html/lenschart.htm. Do not use the APO Tele Converter EX on top of another APO Tele Converter, or any other kinds of converters. Exposure: Using the APO Tele Converter EX decreases the effective aperture by one F-stop (with 1.4x Tele Converter) or by two F-stops (with 2x Tele Converter). However, exposure can be determined in the same manner as when using the master lens without a Tele Converter. Main Specification * 1.4X 2X. * Lens construction 5 elements in 3 groups 6 elements in 5 groups. * Focal length 1.4x that of a master lens 2x that of a master lens. * Aperture coupling range F2.8 - F32 F2.8 - F32. * Reproduction ratio 1.4x that of a master lens 2x that of a master lens. * Weight 143g (5oz.) for the 1.4x and 234g (8.3oz.) for the 2x.