The objective lenses are the front lenses. The diameter of the objective lens is expressed in millimeters. As with the aperture of a camera lens, the size of objective lenses determines the amount of light that can enter your binoculars. It combines with magnification, exit pupil, type of coating and type of prism to determine overall light transmission efficiency. In identifying a particular model, say the Zeiss Diafun 10x30B MC, the second number refers to 30mm objective lenses.