You can use magnification in one sentence

Magnification and reproduction scale

The total magnification of the microscope is calculated from the scale number of the objective, multiplied by the magnification of the eyepiece and, if necessary, multiplied by intermediate magnifications.

There is a difference between magnification and reproduction scale.

While the magnification always relates to the impression of the eye, the image scale is always a measurable variable. If one looks at an object with the eye from a distance of 250 mm, one speaks of a magnification of 1x. If this distance is 500 mm, the object can only be seen half as large - the magnification is now only 0.5x. The decisive factor for this is the viewing angle from which you can see something. When looking into the microscope, the viewing angle from which you can see the object is enlarged - precisely by the factor that results from the above calculation for the total magnification. This does not mean that you can also image the entire viewing angle in the eye! The viewing angle is limited by the field of view.

The image scale always comes into consideration when you create an image that you want to measure with a scale. For example, you take a picture with a microscope camera and want to know how big the object detail was in the specimen. To do this, measure the detail in the photo with a ruler. If you divide the measured length by the total magnification, you get the original size. Example: 10 mm object size on the paper image, photographed with a 40x objective, 10x photo eyepiece, camera magnification 0.25x and enlarged 4x using the negative.

10 mm

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40 x 10 x 0.25 x 4

The result is 0.025 mm or 25 µm. The object detail was therefore exactly 25 µm. The reproduction ratio is 400: 1. If you look at the image from a distance of 250 mm you can see it 400x larger than if you were looking at the object detail on the slide with the naked eye.