An optical apparatus includes an optical combiner, an image lens, and an external scene lens. The optical combiner has an eye-ward side and an external scene side and includes a partially reflective diffraction grating that is at least partially reflective to image light incident through the eye-ward side and at least partially transmissive to external scene light incident through the external scene side. A first mount is positioned to hold the image lens in an optical path of the image light to apply a first corrective prescription to the image light. A second mount is positioned to hold an external scene lens over the external scene side of the optical combiner to apply a second corrective prescription to the external scene light. The optical combiner combines the image light with the scene light to form a combined image that is corrected according to the first and second corrective prescriptions.