Abstract: An ultra-short and novel optical system, designed for the binocular described in U.S. Pat. No. 4,488,790, granted Dec. 18, 1984, having an air-spaced objective that is as short as is physically possible and an Erfle type eyepiece that is also as short as is physically possible, to produce a magnification of 7 power in a binocular with an objective 30 mm in diameter and an eyepiece with a real angle of 9.degree.. A feature of this optical system is the employment of oval mirrors of four different sizes, arranged so that the end mirrors are virtually touching the adjacent lens elements of the objective and eyepiece. Another feature is that the focal plane is curved rather than flat as in conventional binoculars. A further feature is that the first and last achromats of the eyepiece are tilted very slightly at the top in the direction of the objective to elongate the apparent depth of focus by arraying sharp focus along a gradient in the field of view from top (distant) to bottom (near).