Optical extinction of silver nanostructures produced by electron beam lithography has been modeled by a finite element method. While the simulations of the design geometry produce results that are markedly at odds with the experiment, subtle variations of the shape, such as rounding of structure edges, are able to bring the simulations in agreement with experiment. We present the effect of various aspects of structure geometry on the evolution of electromagnetic field modes. They are highly localized at the sharp edges and sides and strongly affected by minor shape variations such as edge rounding. This work demonstrates that optical absorption/extinction provides a simple and sensitive indication of subtle morphology features in metal nanostructures.