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Buying a Microscope

A good microscope is an important prerequisite for observations and examinations. A suitable model should be upgradeable with additional optics. Low-cost microscopes reach the limit of their capacity rather quickly and especially children will soon lose the pleasure in this interesting topic. In the beginning an ordinary, low-cost optics is enough for an upgradeable stand. A 10x and 40x objective and a 10x eyepiece (makes a total magnification of 100x resp. 400x) usually meet with the requirements of a beginner.

Optical System

Here are just a few important parameters: the objective is made of achromatic lenses which create a realistic image without colored fringes. The lens system facing the eye is called “eyepiece”.
The resulting image is magnified but inverted. This means that you have to push the sample to the right to bring an area on the right side of your sample into the center of the field of view. Likewise you have to push the sample downward if you want an area on the bottom of your sample moved upward. Sharp images are only possible with exact centering. In other words, the sample has to be exactly in the middle of the light path.


The total magnification of a microscope is the multiplication of the magnifications of eyepiece and objective. A 10x objective and a 10x eyepiece give a 100x magnification. The magnification capacity of objectives and eyepieces depends on their focal length. The shorter the focal length, the higher the magnification of an optical system.


Much more important than the magnification is the resolution because it decides the value of a microscope. The resolution capability of an optical system is the smallest distance between two points at which they are still distinguishable as separate. With a good microscope a resolution of 0.2 microns can be achieved (1 micron = 1/1000 mm). Generally the objective is the decisive factor for the resolution capability. An eyepiece with a high magnification does by no means increase the resolution but only creates a very faint image and heightens the danger of optical illusions.

Numerical Aperture

The numerical aperture is indicated on every objective. The rule of thumb is: the total magnification should never exceed the 1000-x numerical aperture of the objective being used.
For example: with a 100x objective with a numerical aperture of 1.3 no eyepiece higher than 13x should be used. Moreover, the total magnification should never fall below the 500-fold numerical aperture of the objective being used because thus the resolution capability will never be fully utilized.
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