Prisms of Binoculars Explained

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Every pair of binoculars will have the same basic components. There are objective lenses that allow the light to enter the binoculars and eyepiece lenses that enlarge the image you are viewing. Between these two types of lenses are prisms and the prisms of binoculars happen to be a very important component.

The function of the prism is to give the image you are looking at the proper orientation. Without the prisms of binoculars, looking through a pair of binoculars would result in viewing an image that is upside down and laterally reversed. It is the job of the prism to flip the image over and make it appear upright. Most binoculars come in one of two types of prism designs, the roof prism or the Porro prism.

Porro prism binoculars

The Porro prism gets its name from its inventor, Ignazio Porro who invented this design in the mid-19th-century. Porro prism binoculars have offset lenses, giving the binoculars the shape of the letter Z. Some people say the shape resembles a dog’s leg. This design is what many people envision when they think of binoculars and it is the more traditional design.

Roof prism binoculars

Roof prism binoculars are designed in a way that allows the objective lens and the eyepiece lens of the binoculars to be in line with each other. Because of this, roof prism binoculars are more compact and lighter weight than the more traditional Porro prism binoculars. The lighter weight and more compact design comes at a cost however. Image quality in roof prism binoculars is often not as good as it is in Porro prism binoculars because less light makes its way to the eyepiece. Roof prism binoculars must also be aligned with a great deal of precision which makes them more difficult to manufacture. Being more difficult to manufacture also makes them considerably more expensive than Porro prism binoculars.

The ability to allow light into the binoculars and through the eyepiece lens in roof prism binoculars has been greatly improved by the use of special coatings on the optics so the image quality between today’s roof prism binoculars and the more traditional Porro prism binoculars is negligible in many cases. When shopping for a pair of binoculars the main difference between the two types of prism designs is that the Porro prism binoculars are bigger and heavier but the roof prism designs are considerably more expensive. When considering the prisms of binoculars and trying to decide which type to buy you’ll have to decide if a lighter weight, more compact design is worth the extra money.