An increasing number of microscope websites advertise microscopes with 2,000x magnification, which is strange since 2,000x is double the magnification for which standard light microscopes are designed. Regretfully, since most Consumers are not that familiar with microscopes, there is a natural assumption that 2,000x must be bigger and better than 1,000x magnification. This, however, is not the case.
On further investigation, it appears that the websites in question achieve 2,000x by the simple expedient of including a set of 20x eyepieces with the microscope. Since magnification is achieved by multiplying the power of the objective lens by the power of the eyepieces, then 20x eyepieces x 100x objective lens = 2,000x total magnification, right?
Well, not exactly! A light microscope is designed round the concept of Maximum Useful Magnification. Maximum Useful Magnification (MUM) is approximately equal to the size of the numerical aperture (N.A.) multiplied by 1,000. So, for a microscope with an NA of 1.25, the MUM is approximately 1,250x.
That is why all standard light compound microscopes are designed and sold with 10x eyepieces as standard and 100x objective lens as the largest objective lens. At a 1,000x magnification, you do get higher magnification and improved resolution over, say, 400x, because the total power of magnification does not exceed the MUM of the microscope.
Anything above this maximum produces what is known as False or Empty Magnification. The additional magnification above the MUM yields no further useful information or finer resolution of detail. Quite the contrary. You will likely experience severe degradation in resolution. To quote Nikon, “… excessive magnification introduces artifacts, diffraction boundaries and halos into the image that obscure specimen features and complicate the interpretation of visual interpretations.” Quite!
In plain language, the total magnification of the image increases, but the resolution of the image will degrade to the point where it is useless. The image gets blurry. A good illustration of this principle can be found anywhere online. Have you ever tried to enlarge a PDF file online. The text is magnified when you zoom in, but the resolution does not improve. Bigger and blurrier.
It is true that there is some leeway in maximum useful magnification. For example, with better quality microscopes you can get away with using 16x eyepieces. 1,600x is not such a stretch from 1,250x when using a microscope with NA 1.25. But 20x or 25x eyepieces? Not only will they not work efficiently, but they are likely to frustrate your experience. A case of both false magnification and false advertising, perhaps?