General knowledge of microscope objectives

During the observation of the microscope, the lens aimed at the standard sample is called the objective lens. The imaging quality of the microscope depends largely on the quality of the objective lens, so the objective lens is the most important part of the microscope. In recent years, due to the rapid development of computer technology, the design and manufacture of objective lenses have been significantly improved and improved.

The main parameters of the objective lens include: numerical aperture, resolution, depth of focus (vertical resolution), working distance and field of view

1. Numerical aperture:

The numerical aperture of the objective lens indicates the ability of the objective lens to collect light. The more the objective lens collects the reflected light at various points on the sample, the better the image quality. Numerical aperture is often expressed in NA

2. Resolution:

The resolution of the microscope refers to the minimum distance between two points on the sample that can be clearly resolved, usually expressed by d. Resolution determines the degree to which the microscope distinguishes details on the specimen. The objective lens of the microscope is to magnify the object into a real phase. The effect of the eyepiece is to magnify this real image again. Can be distinguished. Therefore, the resolution of the microscope depends mainly on the resolution of the objective lens.

3. Depth of focus (vertical resolution):

Depth of focus is the ability of the objective lens to clearly image uneven objects. When the microscope accurately focuses on a certain object surface, if the object surface located in front of and behind can still be clearly seen by the observer, between the farthest two planes The distance is the depth of focus. The depth of focus of the objective lens mainly depends on the numerical aperture of the objective lens. The larger the numerical aperture of the objective lens, the smaller the depth of focus. In the case where the numerical aperture of the objective lens is particularly large, the microscope can have very good resolution, but the depth of focus is very small. Therefore, an objective lens with a suitable numerical aperture should be selected according to needs. When the microscope is used for high magnification observation, due to the small depth of focus, the image can only be clearly imaged when the difference in height of the metallographic sample surface is small, so the sample used for high magnification observation should be shallowly etched.

4. Working distance:

The working distance of the objective lens refers to the distance between the specimen surface and the front end of the objective lens after the microscope is accurately focused. The higher the magnification of the objective lens, the shorter the working distance.

5. Field of view:

The field of view refers to the size of the surface area of ​​the sample observed in the microscope. The field of view is inversely proportional to the magnification of the objective lens. The diameter of the ordinary objective lens for the first real image is generally 18mm, the magnification is 10X, 40X, 100X, the field of view diameter is 1.8mm, 0.45mm, 0.18mm

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