Sigma designed a floating focusing system two moving lens element groups to enhance the optical quality down to macro 1:1. Three low-dispersion glass elements were also used to try and correct chromatic aberrations. The OS letters in the name stand for "Optical Stabilization" and help you keeping you from moving when shooting in low light.And, even if this design is quite heavy and expensive, it performs nicely:ComparisonAs we have not yet tested the Canon and Nikon 150mm, we will here compare the Sigma 150mm with a Canon 100mm instead. Both lenses are generally used for portrait, macro,