An eyepiece is a magnifier with its entrance pupil defined by the lens stop of an objective lens. It is also known as an ocular lens. A typical ocular has a focal length of 25 mm, which is one-tenth of the near point of human vision at 250 mm. An image conjugate of the tube lens serves as the object of an ocular. The object is located at the front focal point of the eyepiece. The angular size of the image is 10 times greater than the angular size of the object at the standard near point. Consequently, the angular magnification of a typical eyepiece is 10X. The angular magnification of an arbitrary eyepiece is defined by Eqs. (2.7) and (3.27).
The entrance pupil of an eyepiece is defined by the lens stop of the objective lens. In a system with finite tube length, the object NA is defined by the diameter of the objective lens stop and the length of the tube. In a system with infinite tube length, a tube lens creates a distant image of the objective lens stop, and the object NA is defined by the diameter of the lens stop and the focal length of the tube lens.
The exit pupil of an eyepiece is an image of the entrance pupil, which is the lens stop. During proper illumination, the exit pupil appears as a bright white disk floating above the eyepiece. The exit pupil should fit within the pupil of a human eye. The distance from the last surface of the ocular to the exit pupil is the eye relief.
The eye relief is an important functional parameter. At short distances of 10 to 15 mm, the eye relief prevents contact with eyelashes. At longer distances of 25 to 30 mm, the eye relief provides space for eyeglasses. A flexible eye cup on the ocular acts as a shield from room light for an operator without eyeglasses. The eye cup is folded backward along the ocular for operators with eyeglasses.