The most critical part of the optical microscope is the objective lens, which is also the most difficult part to design and assemble, the objective lens is assembled by a series of lenses, which is responsible for converging the light emitted by the sample and imaging through the eyepiece, which is the first transmission element through which the light is transmitted from the sample to the phase, which is closest to the object, hence the name – objective lens. There will be some logos on the barrel of the objective lens that represent the information of the objective lens, and today we will understand the meaning of the objective lens logo.

1. Manufacturer: A better commercial objective lens will have the words manufacturer on it, such as ZEISS, LEICA, Nikon, OLYMPUS, etc., which are all common microscope objective lens manufacturers, and the quality of the objective lens is relatively good.

2. Objective lens correction factor: under the manufacturer, there will be some English letters, Plan, APO, Fluor, etc., many beginners can not understand the meaning of these letters, these letters are the objective lens design of the aberration correction level identification, but also include the use of scenarios (such as infrared, ultraviolet and other different wavelengths), it is worth mentioning that different manufacturers will have different ways of writing different standards, but the essence represents the design standard of the objective lens, interested can check it yourself. Optical lens imaging often involves spherical aberration, coma, astigmatism, field curvature, distortion, chromatic aberration and other aberrations, correcting these aberrations is particularly critical to the microscopic objective lens containing multiple lenses, according to the degree of correction is roughly divided into the following levels, as shown in the figure, the nameplate of the microscope objective will be marked with these correction information, the price of the objective will also increase significantly with the increase of the correction level.

Achromatic objective: It is a commonly used objective lens in the laboratory, which corrects perpendicular chromatic aberration for blue light (486nm) and red light (656nm), and also performs spherical aberration correction for light at 546nm wavelength.

Plan Achromatic Objective: The achromatic objective does not correct the field curvature, while the plan achromatic objective corrects the field curvature of the achromatic objective so that the curved field becomes flat.

Fluorite Objective: Named after a mineral fluorite, also known as semi-apochromatic lenses, it corrects chromatic aberration and spherical aberration at more wavelengths than achromatic objectives, and also has better imaging quality.

Plan Fluorite Objective also does not have field curvature correction for flat fluorite objectives, and field curvature correction is performed for fluorite objectives to better improve imaging quality.

Plan Apochromatic Objective Lens corrects chromatic aberration and spherical aberration for more colors of light, almost eliminating chromatic aberration, and apochromatic lenses are all with field curvature correction, which is the best choice for white light imaging.

If you want to correct the field curvature aberration, you need to add a lot of optical lenses to the optical design, as shown in the figure for the comparison of the optical structure of the achromatic lens and the flat field achromatic lens, the production cost of the lens multi-objective lens will naturally increase, but the field curvature correction is very necessary in the design of high-end objectives, especially for the apochromatic objective.

3. Magnification: The objective lens will be marked with magnification: 10X, 20X, 50X, 100X, etc., magnification is one of the main considerations when we choose a microscope objective, but the objective lens we buy is marked with 50X, is it really 50 times magnification in our use?

In fact, the magnification of the objective depends on the focal length of the barrel lens and the objective, and the formula is as follows:

Objective lenses from different manufacturers are designed according to their own microscopes, so the magnification is only indicated if they are used on their own microscopes. What if the objective lens is applied to other microscopes or self-made microscopes? The focal length of each manufacturer’s microscope tube lens is fixed, as shown in the following figure:

According to the formula, we can calculate the focal length of the objective lens of different manufacturers, and the magnification can be calculated according to the focal length of the tube lens of the microscope you are using. For example, the focal length of the Nikon 50X objective is 200/50 = 4 mm, and if it is applied to an Olympus microscope, the magnification is 180/4 = 45X.

4. Numerical Aperture: Numerical aperture (NA) is an important parameter of the microscopic objective, which is a function of the focal length and the diameter of the entrance pupil:

The optical resolution of the microscope objective lens Resolution = λ/2n(sin(θ)), the NA can determine the resolution of the microscope, the larger the NA value, the higher the resolution, the n in NA refers to the refractive index of the working medium, n = 1 in the air, θ does not exceed 90°, so the maximum NA in the air medium does not exceed 1, if you want to improve the NA, you can change the medium, such as immersed in oil.

5. Coverslip thickness: Some biological microscopes need to cover a coverslip on the sample when used, the thickness of which is usually 0.17mm, and the microscope objective lens for this application is designed with this 0.17mm distance correction in mind, and its nameplate will also be marked, as shown in the figure.

6. Finite conjugate & infinity conjugate: The finite distance and infinity conjugate here refer to the two conjugate planes of the object plane and the image plane, the finite and far conjugate microscope objective: the light scattered from the sample is collected by the objective lens to a point, and then imaged through the eyepiece, the finite conjugate microscope has two standards DIN (Deutsceh Industrie Norm) and JIS (Japanese Industrial Standard), as shown in the figure. For these two standard microscopes, there are several lengths to be aware of, Mechanical Tube Length (MTL), Optical Tube Length (OTL), and Parfocal Distance. The three lengths are defined as follows:

Barrel length: The distance from the mounting surface of the objective lens to the mounting surface of the eyepiece, that is, the distance from the flange of the objective lens to the flange of the eyepiece.

Optical barrel length: The distance from the rear focal plane of the objective lens to the middle image plane.

Parisofocal length: The distance from the mounting surface of the objective lens (objective flange) to the plane in which the object is being observed.

Both standard objectives are mounted with RMS (0.7965″ x 36TPI) mounting threads

Infinity conjugate microscope objective: the light scattered from the sample is a parallel beam after passing through the objective, and an intermediate image is formed by adding a tube lens between the objective lens and the eyepiece, this modern microscope design method has one more lens than the traditional microscope design, but it makes the configuration of the microscope more flexible, and the distance between the objective lens and the tube lens changes, and the magnification is not changed, and it is not limited by the previous distance, which is convenient to expand the imaging module such as fluorescence and the filter in the microscope, and now the microscope system mostly adopts the infinity conjugate microscope。