Microscope Magnification and More

If you can't reel off the differences between magnification, resolution, and numerical aperture in 60 seconds or less, no worries: you're not the only one. Here's a quick guide to these microscope terms and why they're important.

Microscope Magnification
Calculating the actual magnification of a microscope is literally as easy as doing your multiplication tables. On a compound microscope that has a rotating turret or nosepiece, each objective lens has the magnification rate engraved on its barrel. To get the other piece of the puzzle, check the eyepiece magnification at the top. Then multiply the objective magnification by the eyepiece magnification, and that's the total magnification for that eyepiece/objective combo.

Microscope Resolution Resolution isn't quite as intuitive as magnification. Resolution refers to clarity or fineness of detail rather than size. It's the ability of a microscope lens to delineate separate images of lines or dots. Resolution is determined solely by the objective lens, because a microscope's eyepiece serves only to magnify an image, not to resolve it. While magnification is important, anything higher than 1000x isn't really helpful, because the wave nature of light places a theoretical limit on a microscope's resolution. The resolution limit is about 250 nanometers- small enough for most anything you'll be looking at. Electron microscopes can achieve a magnification level of up to 2,000,000x. However, buying a high-quality one would probably require taking out a second mortgage; they cost well into six figures- or in some cases seven.

Numerical Aperture
Numerical aperture is the ability of a lens to increase resolution by collecting a wider cone of light from the specimen. It's directly proportional to both resolution and magnification: the higher the magnification, the higher the numerical aperture.

High-powered objectives need more light the closer they get to a specimen, and the numerical aperture- in essence, the hole through which light passes- must therefore be higher. In the case of high-power microscopes, the numerical aperture is about 1.4. With low-power microscopes, the number is more like 0.04. The numerical aperture number can usually be found engraved next to the magnification on the barrel of the objective. It's the number that's not followed by an X.

A microscope's light-concentrating condenser also has a numerical aperture; the condenser's number must be equal to or greater than the objective lens' numerical aperture or the specimen won't be sufficiently lit. Accordingly, high-powered microscopes require microscope condensers with high numerical apertures.