17-50mm fixed f/2.8 aperture. SP - Super Performance (Tamron's highest quality lens line) AF - Autofocus XR - Extra Refractive Glass (reduces lens size) Di - Digitally Integrated Design (to reduce ghosting, flare, CA and peripheral light fall-off along with improved resolution) LD - Low-Dispersion lens elements (to reduce chromatic aberration for sharper images) Aspherical - contains aspherical elements (compensates for spherical aberration and distortion, making for better performance with fewer elements (and thus allowing for smaller, lighter lenses) (IF) - Internal Focusing Must for Portrait Photography