Let’s start off by taking a look at how these two lenses compare when mounted on a Canon 5D Mark II body:Sigma 12-24mm F4.5-5.6 EX DG HSM II Canon vs Sigma 12-24mm F4.5-5.6 EX DG CanonThe most striking improvement for the EX DG HSM II is with respect to chromatic aberration, thanks to the use of FLD (“F” low dispersion) glass, specially formulated to simulate the transmission properties of fluorite glass (i.e., lower refractive index and dispersion as compared to other optical glass). There is marked suppression of chromatic aberration at 12mm, particularly in the image center,