Binoculars today (and most spotting scopes) are available in two general profiles, or shapes. These shapes are a function of the type of prism system used in the instrument.
Porro prism binoculars have the classic "dog leg" shape with the eyepiece offset to the one side with respect to the objective or front lens. These are like a great horned owl—wide, bulky, and stout. They provide a big, steady platform for observing and offer a nice hand-filling package, especially for people with large hands.
By contrast, roof prism binoculars have a straight profile, with the eyepiece situated directly behind the front lens. These are more like a harrier—trim, narrow, and straight. This allows them to be held with arms tucked close to the body for maximum steadiness. The narrow profile of a roof prism also places focusing wheels conveniently under the fingers, which enhances focusing speed and comfort, especially for people with smaller hands.
The difference between these two prism types is obvious when you handle them, and many shoppers choose one or the other largely on the basis of comfort and cost. This is fine for the casual user, but performance-conscious birders, especially those with a limited budget, need to take a closer look at prism type. There are optical differences.
The function of a prism in a binocular or spotting scope is to provide a properly oriented image. Prisms are expensive, but without them a binocular would always produce upside-down, reversed images, challenging even the most skilled birder.
Porro prisms, the older design, are 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 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.
Spotting scopes also use Porro and roof prisms, but the overwhelming majority are Porro prisms, because size is not such an issue with these instruments. There are some exceptions, notably small, compact roof prism scopes designed for maximum portability, but these do not offer interchangeable eyepiece capability. With the exception of large European drawtube scopes, all the premium, full-feature, large-objective scopes on the market today use Porro prisms.
There are other optical systems used in spotting scopes that deserve attention. These dual-purpose instruments can be used for birding by day and for astronomy by night.
Small, high-quality astronomical refractors are becoming popular with birders for their versatility and exceptional optics. This type of scope is basically the same as a standard Porro prism spotting scope, but the Porro or erecting prism is detachable (not needed for astronomy). In addition to superbly corrected optics, these scopes offer virtually any combination of eye relief, magnification, and field of view, as astronomical eyepieces are all industry standardized according to a few basic barrel diameters. Thus, any brand of scope here can use eyepieces from a dozen or more manufacturers.
There are also small, compact scopes on the market that use a combination of mirrors and lenses. These are technically known as catadioptric scopes, and there are several design variations. Most of the smaller ones are the Maksutov design, which requires a high degree of workmanship to produce a decent optic. Traditionally, this has meant a very high price tag, but new computerized production techniques have produced some new models at reasonable prices.
The chief advantage of these scopes is their short tube length relative to their aperture. This makes them amazingly compact, although they are not as rugged or as durable as typical Porro prism spotting scopes. They do, however, use astronomical eyepieces and photographic adapters, which makes them very versatile observing tools.