Eyepiece is a visual optical device used to observe the image formed by the front optical system. It is an integral part of visual optical instruments such as telescopes and microscopes. Its main function is to enlarge the real image magnified by objective lens again. In order to eliminate aberration, the eyepiece is usually composed of several lenses, which has a large field of view and viewing angle magnification.
 
The eyepiece is also the main component of the microscope, and its main function is to enlarge the real image magnified by the objective lens again, so as to form a clear virtual image at the bright distance; Therefore, its quality will finally affect the quality of the object image. In microphotography, a real image is formed at ground glass. Some eyepieces (such as compensation eyepieces) can not only amplify, but also correct the residual aberration produced during the imaging process of objective lens. The structure of eyepiece is much simpler than that of objective lens. Because the light beams passing through the eyepiece are nearly parallel, spherical aberration and longitudinal (axial) chromatic aberration are not serious. Only transverse chromatic aberration (amplified chromatic aberration) is considered in design. The eyepiece consists of two parts, and the lens at the upper end is called the eyepiece lens, which plays an amplifying role; The lower lens is called converging lens or field lens, which makes the image brightness uniform. A light bar is arranged in the middle of the upper and lower lenses or at the lower end of the lower lens, and accessories such as micrometer, cross glass and pointer are installed here. The aperture angle of the eyepiece is very small, so the resolution of the eyepiece itself is very low, but it is enough to enlarge the initial image of the objective lens. The common magnification of eyepiece is 8 ×, 10 ×, 12.5 ×, 16 × and so on. It is usually equipped with 2-3 eyepieces at the upper end of the lens barrel, which are engraved with 5 ×, 10 × or 15 × symbols to indicate their magnification, and are usually equipped with 10 × eyepieces.