Field diaphragms for use in surgical microscopes are provided. The field diaphragms are positioned along an optical axis of a microscope illumination system. The field diaphragms include a frame portion configured to be received by the surgical microscope; and a non-circularly symmetric mask portion integrated with the frame portion. The mask portion is aligned such that marginal rays from an edge of the field diaphragm along a meridian of minimum diameter that reflect from a surface of an objective lens of the microscope reflect outside of an acceptance angle for relay through any ocular channel of the microscope.