Optical apparatus includes an image sensor and an optical assembly, which is configured to focus optical radiation via an aperture stop onto the image sensor. The optical assembly includes a plurality of optical surfaces, consisting of a first, curved surface through which the optical radiation enters the assembly, a final surface through which the rays exit the assembly toward the image sensor, and at least two intermediate surfaces between the first and final surfaces. An interference filter, which has a center wavelength and a passband no greater than 4% of the center wavelength, and includes a coating formed on one of the optical surfaces. All rays of the optical radiation passing through the aperture stop are incident on the coating over a range of incidence angles with a half-width that is no greater than three fourths of the numerical aperture of the optical assembly.