Porro prisms are generally physically wider than long. When viewed from the side, they direct light along a path that is folded into something akin to a square S shape. If the prisms are of quality glass and aligned correctly, there is very little light loss or degradation of the image. The only real disadvantage is the fact that the prisms are large and bulky and consequently require large housings.

Roof prisms, on the other hand, are a newer design. They are smaller and more compact than Porro prisms, and actually resemble a small house with a peaked roof. This allows them to be fitted into smaller housings, which makes for more compact binoculars or spotting scopes.

However, standard roof prisms have several inherent design difficulties. First, they are (generally) not as bright as Porro prisms because they employ mirror surfaces that lose some light. Second, alignment is often more critical than in Porro prisms; an alignment error a fraction of the width of a human hair will degrade the image in a roof prism. Securing them in binoculars is much more of a challenge. Third, images are split and then rejoined slightly out of phase when they pass through such a prism. This produces an image of slightly less resolution than an equal quality Porro prism.

Thus we have a trade-off: the slight optical superiority of the typical Porro prism versus the comfort factor of the standard roof prism. Yes, average quality Porro prisms tend to give a sharper image than standard roof prisms. There is a price to pay for comfort.

Technology, however, has come to the rescue of the roof prism design. Enhanced coatings on some roof prism mirror surfaces reduces light loss to negligible levels. And even moderately priced roof prisms are now housed in cages that make them more secure than many Porro prisms. Best of all, some new roof prisms have special coatings that eliminate the phase problem. These phase-corrected or PC roof prisms can deliver images on a par with the best Porro prisms.

That’s the good news. The bad news is that all this extra effort and attention doesn’t come cheap. PC roof prism binoculars from Bausch & Lomb, Zeiss, Swarovski, and Leica are very expensive, often more than $1,000, but they rank as some of the finest instruments made. They are truly the standard of excellence in binoculars today. If you are a roof prism fan, you can now have your cake and eat it too if you can afford it.

If you can’t, standard roof prism models of at least average quality are still a good choice for birding, as long as they have all the other features of a good birding glass. But if you demand the best in resolution and sharpness, keep in mind that these roof prisms can only do so much without phase correction. A birder whose first concern is resolution must therefore pay the price for a PC roof prism, or opt for a good Porro prism.

In fact Porro prism binoculars remain the best buy for your optical dollar, because they can do anything a PC roof prism binocular can do, and at a fraction of the cost. It’s just plain economics—it costs less to produce a Porro prism binocular to the same optical standards as PC roof prism binoculars. Consequently, if you see two binoculars at the same price, one a Porro prism and one a roof prism, it shouldn’t be too hard to predict which one has better optics.

This is especially important for performance-conscious birders on a tight budget. Inexpensive Porro prism binoculars today are much better optically than inexpensive roof prism models. If you must buy cheap, at least go with a Porro.

But keep in mind that whatever optical deficit roof prism designs once had, the improvements in roof prism optics such as phase-corrected coatings have mostly erased them. And in the marketplace, where consumers vote with their wallets, birders who are buying top-end binoculars tend to elect roofs over Porros. This is why almost all the new advances in birding binoculars have been in roof prism designs.