The invention provides an optical instrument including means (1, 2) to produce an optical image to be viewed by an observer including a diffractive element (7) located at an intermediate focal or image plane (4) of the optical instrument and comprising a pattern of a plurality of areas effective to cause diffraction interference in light passing through the optical instrument and thereby produce an expanded exit pupil (6) comprising a combination of a multiplicity of exit pupils displaced relative to one another transverse to an optical axis (3) of said instrument.