Abstract: The field of the disclosure is that of enlarging optical lenses and, more precisely, binocular optical systems designed to be inserted into the night-time channel of an observation system, between a light intensifier tube and an observer's eyes. The disclosure can be applied especially to observation instruments for armored vehicles and, notably, periscopes for tanks. The disclosed optical system for a binocular device is of the type constituted by two groups, a forward group and a rear group, the rear group being constituted by a bonded doublet with plane rear face comprising a convergent convex lens bonded to a divergent lens, the lenses of the bonded doublet having different and complementary dispersions to compensate for the chromatism, and the focal length of the doublet ranging from 0.85F to 1.05F, where F is the focal length of the binocular device.