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Light Microscopy: Instrumentation and Principles

Author(s): David R. Caprette, PhD

“Empty” Magnification

The best quality (and very expensive) magnifying glasses may produce reasonably good quality images with up to about 25x magnification. We can obtain magnifications up to 40x or maybe a bit more using what is called a stereomicroscope or dissecting microscope. Dissecting microscopes are designed to provide room to work on a specimen while looking at a magnified image. The objective lens may bring the image into focus when it is three or four inches above the specimen.

Why can't a dissecting microscope be used in place of a compound light microscope? Suppose your dissecting microscope can produce up to 40x total magnification when used with 10x eyepieces. Suppose that you find an eyepiece rated as magnifying 20 times. You will make the apparent image twice as large, but you won't see any more detail. This kind of magnification, in which we try to enlarge images beyond a practical limit, is called "empty" magnification. There is no benefit to making a blurry image larger.

We encounter a similar situation when we enlarge a digital image. We produce empty magnification once we reach the point at which the eye can distinguish individual pixels (somewhat less than 200 pixels per inch). Resolution is determined by the number of pixels that define the image, regardless of the physical size of the image. Enlarging pixels themselves does not improve resolution. In a microscope, resolution is limited by the quality and design of the lenses, by the medium in which light travels, and ultimately by the wave nature of light itself.