Roof prism binoculars are better, optically, than the older porro prism design. Right? Must be true because roof prism binoculars typically cost more than porro prism binoculars and all the top dollar binoculars are roof prisms. Fact or myth?

For those who don't know the difference between roof prism binoculars and porro prism binoculars, you can recognize a roof prism binocular by its straight barrel body design and porro prism binocular by its offset body design. For more on the design specifics, see my article at How to Understand Binoculars.

Roof prism binoculars offer a slender body design, which most people find more comfortable to hold and roof prism binoculars also typically offer smoother focusing and, since the focusing mechanism is inside the body and not exposed to the elements. For this same reason, the focusing mechanisms on roof prisms binoculars also typically last longer.

All these roof prism binocular advantages come at a cost, however. An image entering a roof prism is split and when the image is rejoined on the other side, it will be out of phase - some of the wavelengths of light that carry the image are not rejoined precisely. As a result, image sharpness will suffer. The cure for this is a special coating called phase coatings. All but the cheapest roof prism binoculars are now phase coated.

Another optical hurdle that must be overcome with a roof prism is loss of light at a mirror surface which reduces image brightness. In order to combat this, the best roof prisms are coated with ultra high and expensive mirror coatings.

How do porro prisms combat these problems? They don't have to. Porro prisms don't slit the image, so there is no phase problem and they don't use a mirror, so there is no loss of light at a mirror surface. Furthermore, alignment of the prisms is far less critical in a porro prism binocular.

Bottom line is that it is more expensive to make a good roof prism to the same optical standards as a good porro prism. Optically, you get your most bang for the buck with the porro. If you are on a budget and optics are your primary concern, go with a porro. Under $200, I have yet to find a roof prism model that will compare to a porro such as the Nikon Action Extreme 8x40 binocular or Pentax PCF WPII 8x40 binocular and at a bit over $300, the Swift Audubon 8.5x44 porro prism binocular will give many high end roofs a run for their money. Lastly, I will put the Nikon Premier SE 10x42 binocular against any binocular made at any price for its superb optics.

Myth busted.