A method has been developed to simulate the effects of scattered light on the image quality of optical systems. The coherent model is based on geometrical optics to take account of wavefront aberrations caused by lenses, applies finite-element calculation to solve Maxwell's equations around small scattering structures such as edges of diffractive surface zones, and uses scalar diffraction for free-space light propagation. The implementation is discussed in detail, and the operation is demonstrated on diffractive intraocular lenses. Point spread and modulation transfer functions are evaluated for an axial object point, taking account of scattered light as a function of slant angle and round radius of diffractive zone edges. Results show that, at a distance of ±200 Airy radius (i.e., ±2.1∘) from the axis, scattered irradiance is about 5 times more than without considering edge effects. Optimum round radius was found to be 7% of the step height, which agrees with simple geometrical optical estimations.