A prism can be made from any transparent object, but in optics they are most commonly made of glass. The simplest prisms are triangular and must have at least two smooth faces at acute angles. These angles create a color spectrum by absorbing and dispersing light. Prisms in optics lengthen the light path from the objective lens to the eyepiece, increasing magnification without increasing the binocular tube length. Optics are typically made with BAK4 prisms, a German-invented glass that improves refraction. They create an optimal amount of light travelling through your binoculars and into your eyepiece.
Porro prisms, named for Italian inventor Ignazio Porro, consist of two right-angled prisms facing each other. In a Porro prism device, the eyepiece is not in line with the objective lens, so the prism must jag the light sideways in its path across the two prisms. Binoculars were originally built this way to increase the pathway length of the light and optimize image quality.
Different styles of roof prisms can be found in binoculars, but the most common types are the Abbe-Koenig and the Schmidt-Pechan. The Abbe-Koenig is comprised of two connected prisms that form a V-shape. When light enters the prism, it bounces across several angles before exiting through the opposite side, extending the light’s path. Schmidt-Pechan prisms are slightly more compact. They are also constructed of two prisms, but these are separated by a slight air gap. The unusual angled shape of the Schmidt-Pechan means the light path can travel longer than the Abbe-Koenig prism, improving brightness and light absorption.
While you can find a decent pair of Porro prism binoculars for under $100, you’re more likely to pay between $100 to $200 for a good quality device. If you want a decent pair of roof prism binoculars, however, plan to spend between $200 to $1000 dollars on average.
If you’re planning to do your glassing in heavily forested areas or other places where obstacles are close, you may want to choose Porro prism binoculars. You’ll get a wider field of view and better clarity within your glassing range for less money. This is ideal for birders and hunters. For longer-range wildlife watching or stargazing, you’ll want more magnification. If you fall in this category, the higher power range of the roof prism style will suit you better. But you’re likely to find a great pair of binoculars in either style. Just remember to consider your budget and what you’ll be glassing before you purchase.
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