Tamron XR (extra refractive index glass) technology helps give Tamron lenses better performance and allows them to have more compact designs.
Extra Refractive Index glass bends light at steeper angles, and this allows for a decrease in the actual length of the lens. At the same time, it enhances imaging performance by minimising optical aberrations.
Using this technology, Tamron has been able to develop lenses with upgraded image quality without sacrificing lens speed and at the same time making lenses with a smaller diameter which are lighter.
XR glass enables the lens to have the same light-gathering ability with a shorter lens barrel. It also allows Tamron to provide greater zoom ranges in lenses that are more convenient to carry and hand-hold.
Tamron lenses which incorporate XR technology include the AF 28-300mm f/3.5-6.3
and the SP AF 17-50mm f/2.8 VC XR Di II LD