As concentrating solar power systems push towards higher temperatures and lower costs, it is critical that losses of overall system performance can be attributed correctly to the appropriate source. Up to now, this has been poorly done for the case of optical errors, since applicable methods do not exist to quantify how much different imperfections contribute to reducing the upper-bound efficiency of the overall system. Here, the exergy impact of varied optical design parameters-slope error, rim angle, mirror reflectance and sun-shape-is calculated for the first time. Slope error is shown to have the strongest impact. Also, dishes with rim angles significantly wider than the conventional 45 degrees are shown to yield the best overall energy conversion. The resulting analysis method, broadly applicable in concentrating solar power, enables a new approach to quantitative optical system design.