forms the primary image of the microscope which is seen through the eyepiece. The markings on the objective lens are the magnifying power (such as 10x), followed by the NA (0.25) and the tube length. Other numbers which appear on the objective lens may refer to the manufactures catalog number of the particular item.
The objective or front lenses are the lenses located furthest from the user's face during viewing. The diameter of one of these lenses, given in millimeters, will be the second number describing a particular binocular. Hence, a 7x42 binocular has an objective lens of 42mm. The diameter of the lens determines the light gathering ability of the instrument. The larger the lens is, the greater it's light gathering capabilities. Light is essential to image detail and clarity. Therefore, the greater the light gathering capability of a unit is, the clearer and brighter its picture. This fact might lead you to assume that bigger is always better when it comes to the diameter size of the objective lenses, but in reality the size of the lens must be considered along with magnification, exit pupil and intended usage.