An optical instrument for producing an optical image to be viewed by an observer, the optical instrument comprising: an optical system for producing an optical image of an object which is viewable by an observer at an exit pupil; and a diffractive element located at an image plane of the optical system for producing an array of the exit pupils, which are perceivable as a single, enlarged exit pupil by the observer; wherein the diffractive element comprises a surface which has an array of diffractive units, each of which generates one of the exit pupils of the array of exit pupils, the diffractive units each comprising a lenticule, which is effective to produce diffractive interference of light and generate a plurality of exit pupils which are displaced relative to one another in the form of an array of exit pupils, the lenticules each comprising an irregular feature having a plurality of surfaces; wherein the diffractive units are disposed progressively radially outwardly from the optical axis of the diffractive element and configured progressively to provide for an increasing angular offset, such that, independent of location on the aperture of the diffractive element, light from the received image is relayed to a common region on a viewing plane across the aperture of the diffractive element.