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False Claims - Empty Magnification


Empty claims - false magnification or is it the other way round? We have all visited microscopy websites that advertise "1,600x" or '2,000x" compound microscopes and "90x" stereo microscopes, but what they are really advertising is Empty or False magnification that is mostly useless.

Anyone looking to buy a microscope knows, or quickly learns, that the total magnification of a microscope is arrived at through the simple expedient of multiplying the power of the objective lens by that of the eyepiece. So a 10x objective plus a 10x eyepiece = 100x magnification. And a 100x objective lens with 20x eyepieces = 2,000x magnification - right?

False! False magnification, that is. 

False magnification is when the power of the eyepieces employed pushes the maximum useful magnification above 1,000 times the numerical aperture (N.A). For example, you can achieve 1,000x magnification by using a 40x/0.65 N.A with 25x eyepieces. However, the total magnification of 1,000x exceeds the value of 0.65 N.A multiplied by 1,000 (1000 x 0.65 = 650).

As a result, while your image will be magnified 1,000 times, it will yield no further useful information or finer resolution of detail. Quite the contrary, you will likely experience significant to severe degradation in resolution. The image becomes blurry in much the same way as when you try to zoom in on a webpage. It gets bigger, but there is no improvement in the resolution; no improvement in the amount of detail you can see. To quote Nikon, "In fact, excessive magnification introduces artifacts, diffraction boundaries, and halos into the image that obscure specimen features and complicate the interpretation of visual observations.

OK! Now use a 100x objective lens with an N.A of 1.25 and 10x eyepieces. You achieve the same level of 1,000x magnification. The difference is that not only do you achieve higher magnification, but you also benefit from improved resolution. In other words, you can see materially better details in the image. Why? Because you have not exceeded the maximum useful magnification of 1000 x N.A, which in this example is 1000 times 1.25 N.A (1.25 x 1,000 = 1,250).

If you were to use a microscope camera with the two different objective lens/eyepiece combinations, you would see an even greater difference in clarity and detail between the two solutions.

Having read this far, it should come as no surprise to know that every compound microscope is designed and sold with 10x eyepieces as the standard benchmark. There is minimal benefit in using higher power eyepieces and considerable disadvantages. As if this were not enough, there are very few and rarefied applications in light microscopy that actually warrant higher magnification than 1,000x. By the same token, over 90% of stereo or low power applications employ magnifications of less than 45x.

So why do you need higher power eyepieces. Typically, you do not and you should not be bamboozled into paying extra for microscopes with additional eyepieces unless you have a specific requirement to isolate something in the field of view. For example, you may wish to measure an specific element in a smaller field of view using a reticule. In other words, there are some applications that may warrant higher power eyepieces. For general purpose use, however, they are at best not required and at worst, a waste of money. With that in mind, we sell just one microscope, the OM88, with additional 16x eyepieces. This is a popular microscope with doctors and clinics and the 16x eyepieces operate at the maximum useful magnification for the 40x objective lens.

Table 1 summarizes the common objective/eyepiece combinations that lie in the range of useful magnification.

Table 1: Range of Useful Magnification

* These two objective/eyepiece combinations fall below the Minimum Useful Magnification range. This is usually set at 500 times N.A. However, it is highly arbitrary and all our microscopes operate effectively with 10x eyepieces and both 4x and 10x objective lenses.

The important thing to note about Table 1 is that the higher power eyepieces (15x, 20x and 25x) do not operate effectively in combination with the higher power objective lenses. In other words, it is meaningless to advertise a high power, compound microscope as "2,000x Magnification" since it does not work. In reality, standard, light microscopes are designed for a maximum of 1,000x, please - don't waste your money on claims of higher magnification via higher power eyepieces.

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