Porro or Roof? That is the question!

Have you rated your binoculars yet?

3 January 2010

Arkadiusz Olech

1. Construction

A binoculars consist of three main optical parts: front lenses, prisms and eyepieces (oculars). Lenses conduce to gather as much light as possible. Eyepieces are essential to magnify the image and direct it into our eyes as a parallel light beam. Prisms system is situated between lens and ocular and its function is to provide a properly oriented image. Without them a binocular would always produce upside-down, reversed images.

Binoculars mainly are build basing on two typical systems related to prisms arrangement (see figure below). Porro (dubbed of Italian artillery officer) prisms - the older design are simpler and more popular in cheap instruments. This construction causes that front lens axis is situated in a different line than eyepiece axis. On the one hand we may consider this construction as a disadvantage because binoculars are physically wider. But on the other hand the longer is distance between both lenses, the more tridimensional image is gained. Porro prisms are easy to manufacture and easy to put together inside the binoculars. Optical phenomenon called total internal reflection causes that on the entire prisms system there is very little light loss or degradation of the image.

The second, roof construction is distinguished by arrangement lenses, prisms and eyepieces in the one line. It allows to fit them into a small housing, which make them more comfortable to hold outside. That feature will be surely appreciated by extreme sports lovers, mountain-climbers, hunters or birdwatchers who rate highly dimensions of their optical instrument. Unfortunately there is no such thing as a free lunch. The price for a shorter distance between lenses is higher degree of optical compilation inside the roof-system binoculars. The light beam is split and then rejoined. To avoid phase differences, which may cause less resolution and worse contrast, the binoculars with roof prisms have to be equipped with the highest quality optical elements and special coatings. That’s why the prices of roof-type binoculars, giving the same quality image as porro-type binoculars, are usually 2-3 times more expensive.

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