The lens and objective lens of a microscope. The eyepiece is installed on the upper end of the lens barrel, close to the eyes of people. The objective lens is installed on the converter at the lower end of the lens barrel.

(1) Type of objective lens and identification of marks on metal shell of objective lens

The objective lens is the most important optical component of the microscope. Its role is to magnify the object for the first time and determine the imaging quality and resolution. The resolution mainly depends on the numerical aperture NA (aperture ratio) of the objective lens.

The types of objective lenses can be divided into the following categories according to the degree of aberration correction:

① Achromatic objective

A common objective lens, whose shell is not engraved with a symbol, can focus red and blue light in the spectrum at one point, and yellow and green light at another point. Its optimum clear range is 510-630nm, and its performance is poor, so it is not suitable for photography.

② Apochromatic objective

The shell of the objective lens is engraved with APO, which has high performance. It can focus the red, blue and yellow light in the visible spectrum at one point, and correct the spherical aberration of red and blue light and other aberrations. Its optimal clear range ranges from 400-720 nm, containing all visible spectra. Suitable for observation and photography under any colored light.

③ Fluorite objective is also called semi apochromatic objective

The shell of the objective lens is engraved with FI, and the chromatic aberration correction is between the achromatic objective lens and the apochromatic objective lens. The best clear range is from 430 nm to 680 nm. It includes most of the visible spectrum, and is suitable for observation or photography with various filters.

The above three kinds of objective lenses all cause the plane object to form a curved phenomenon similar to a sphere. As a result, the image of the center and edge of the field of vision cannot be focused at the same time, the center is clear, the edge is blurred or opposite, and increases with the increase of magnification.

④ Flat field objective

PI is engraved on the shell of the objective lens. In addition to the advantages of the general objective lens, the flat field objective lens also corrects the bending of the image field. There is no phenomenon that the center and edge of the field of vision are not aligned at the same time. Because the image is flattened, at the same multiple, the image is larger than that of the general objective lens, which is an ideal objective lens today. There are many kinds of flat field objectives, among which "PI APO" (flat field apochromatic objective) is the best. The flat field objective lens shall be used together with the specially made flat field eyepiece.

Understanding a set of data representing performance on objective lens

It is customary to call the objective lens with a magnification of 10 times or less low power, the objective lens with a magnification of 40-45 times high power, and the oil immersed objective lens with a magnification of 90-100 times oil lens. A group of numbers are usually marked on the general objective lens to indicate the relevant performance indexes of the microscope, and 10/0.25 and 160/0.17 are marked on the low power objective lens; 40/0.65 and 160/0.17 on high power objective lens; 100/1.25 and 160/0.17 on the oil mirror. 10 (40, 100) represents the magnification; 0.25 (0.65, 1.25) represents numerical aperture (aperture ratio); 160 or 170 (mm) represents the length of the lens barrel; 0.17 (mm) indicates the thickness of the required cover glass.

(2) Type of eyepiece

The eyepiece is used to magnify the image reflected by the objective lens for the second time, without increasing the resolution, but can correct the remaining aberration and chromatic aberration of the objective lens. It is generally marked with 5x, 7x, 10x, 15x, 16x and other magnifications, which can be selected as required.

There are two types of eyepiece diameters: standard and wide field

The most commonly used eyepieces are: Huixin eyepiece, Lambsden eyepiece, compensation eyepiece, flat field eyepiece and photography eyepiece.