Because diffraction and aberration affect the performance of the optical system, a new metric is advanced that emphasizes the link between these three aspects. In general, the modulation transfer function is used as a measure for contrast sensitivity reduction, whereas the root mean square error is often used to quantify the optical quality of the system. However, for a given object, wavefront aberrations can alter the output image very differently even though they have the same root mean square error and modulation transfer function profile. A distinction between coherent and incoherent illumination is made, and a new measure, called optical transfer error, is defined to characterize optical performance complementarily to root mean square and modulation transfer function. The optical transfer error measures the optical excellence in terms of the shape of the wavefront as well as the theoretical performance results. Several illustrations are presented to better discuss optical performance.