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How Binoculars Work
Many bird watchers and star gazers know they want a good set of binoculars to give them a clear image of their favorite bird or constellation – but they have no idea what “a good pair” really means. If you’re not sure what you need, or if you want to know a little more about binoculars in general, read our basic primer about how binoculars work before you hit the stores looking for the perfect pair of star gazers.
Magnification and Aperture
The size of binoculars is expressed in two numbers that look like an equation (although they are not an equation). To take an example of a fairly high powered pair of binoculars, 8x40, the first number represents the magnification possible; the second number is a measure of the aperture in millimeters. Aperture means “opening” and here it refers to the size of the objective lens, where light comes in. So, 8x40 has a magnification of 8 times with an objective lens that is 40 mm wide.
Binoculars 101
If you simply want to magnify an image that is far away, a telescope will do just as well as a pair of binoculars. Binoculars are essentially two telescopes mounted together. The advantage of binoculars is that you use both of your eyes which reduces or eliminates eye strain.
At the end of each telescope the objective lens gathers light from the objects you observe - the image is inverted (you can see this inversion by playing with a magnifying glass and any well lit image). In a classic telescope, light is focused and re-inverted through another lens, called the eyepiece. This is the simplest telescope, as seen in the illustration below.
In order to get any significant magnification, you need a very long tube. This is okay if you are using a stationary telescope to view the stars, but is not really acceptable for a pair of bird watching binoculars.
This kind of non-prismatic binocular (seen below) was used long ago.
These were once useful but are now mostly museum pieces, having been outdone by the use of prisms in the binocular barrel.
Prisms
Today, prisms are used to condense the viewed image for maximum magnification in a short space. Prisms recreate an image of the original that is much purer than any image you would get from mirrors or multiple lenses which could magnify the image in a shorter distance. The resulting distortion would render the image impractical.
There are two kinds of prisms used in binoculars: roof prisms and Porro prisms. You can tell which type of prism is used from the shape of the binoculars.
Binoculars with Porro prisms have a “jog” in the light path that results in the eyepiece being much smaller than the barrel of the binoculars, as seen in this diagram:
Roof prisms result in an exterior design of the binoculars that is more like two straight tubes, as seen in this picture:
The roof prism design was perfected in the 1960’s; binoculars with this design are compact and easier to hold. For a while after their introduction, roof prism binoculars required much greater optical precision, so they cost more to manufacture. The Porro prism rendered better contrast and was still favored by purists for a long time after the advent of the roof prism. The technology of roof prisms has improved greatly since their debut, which makes newer models a better value.
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