The Binocular Site
The Power of Porros
Take a casual look at the binoculars for sale most anywhere today, and you will notice one striking pattern—they are predominantly roof prism models. Not originally well received when introduced to the market around a century ago, roof prism binoculars have made tremendous gains in popularity over the past two decades as consumers have come to prefer the slimmer, smaller profile the roof prism design makes possible.
But while it is now somewhat out of favor, the roof prism’s predecessor, the Porro prism, offers particular features and benefits that roof prism models are not as good at providing. For this reason, anyone considering buying new binoculars would do well to add a few Porro prism models into their potential choices.
The Porro prism binocular—named for its inventor, Ignazio Porro—was the first major improvement upon the original Galilean binocular design, which was essentially two telescopes connected together with a hinged bracket. Not a poor design as long as the magnification levels were kept fairly low, the Galilean design suffered from the problem of needing to be made longer (to provide a lengthened optical path for the higher level of magnification) and larger (to increase the overall amount of light needed for that lengthened optical path) if a higher level of magnification was required. Without modern lens design technology or lightweight synthetic materials available for the chassis and other mechanical parts, a larger binocular meant a much heavier one as well.
Porro’s solution to this problem was ingeniously simple: use prisms to bend the light in the optical path of each half of the binocular so that a longer optical path could be contained in a more compact physical structure. Two identical prisms of a simple right triangle shape were used in each half of the binocular. These prisms were positioned midway between the objective and eyepiece lenses, perpendicular to one another and with their longer sides (hypotenuses) facing together at one corner. The Porro prism binocular design bends the light path of each optical channel in the binocular five times, greatly lengthening the distance the light travels without requiring the binocular itself to be made physically longer.
By using triangular prisms and positioning them in such a way, the objective lenses were no longer directly in front of the eyepiece lenses. In the classic Porro shape, the objective lenses are further apart from one another than the eyepiece lenses (which must be kept within a distance that is adjustable to the same distance apart as a person’s eyes). Thus, while the overall length of the Porro prism binocular was reasonably easy to handle and balance, the width was increased to accommodate the Porro prism assemblies.
This greater width was not necessarily bad. By spacing the objective lenses wider than human eyes, the visual image transmitted through the binocular to the user is one with greater depth of field, and of superior contrast. Just as the eyes of a predator, such as a lion, are set closely together in order to concentrate all attention on whatever it selects as its prey, so the eyes of prey animals, such as a gazelle, are set widely apart on its head in order to take in a wider view of the world in order to keep a sharp lookout for predators. Humans, being predators, have forward looking eyes that allow us improved perception of movement and focused vision, but rob us of the more three-dimensional view of the world that enables prey animals to discern the shapes of potentially dangerous predators, even when those predators are motionless. Likely without intending to do so, Porro solved not one but two problems with his innovative binocular design—make a more powerful binocular shorter, and enable that binocular to provide its user with an enhanced three-dimensional image of the object being viewed.
Sadly, the popularity of the roof prism has left all too many classic Porro prism binoculars neglected. In fact, just this year Nikon removed one of its most famous Porro models, the Superior E, from its product line. Fortunately, some fine Porro prism binoculars are still on the market. In fact, one of the finest Porro prism designs ever made is presently available from one of the optical world’s great names: Swarovski. The Swarovski Habicht 7x42mm, 8x30mm, and 10x40mm binoculars combine the classic European binocular design with state-of-the-art optical quality to produce an optical instrument of superb quality, as well as a true work of art in its physical design. Providing the user with an exceptionally vivid, three-dimensional image, Swarovski Habicht binoculars can bring out the subtle details in objects that most other binoculars on the market today simply cannot.
Taking the power to produce a three-dimensional image to unprecedented levels, the Opticron HR WP 8x42mm and 10x42mm binoculars are designed with exceptionally wide spacing between the objective lenses for one of the widest image triangulations of any binocular design presently available. Thanks to some creative engineering, these binoculars’ comfortable and ergonomic design allows them to be well balanced and very easy to use.
Steiner offers lines of military, marine, and law enforcement binoculars, nearly every one of which is a Porro prism model to provide those users with the ability to see objects or people that are intentionally trying to remain hidden. The company also offers the innovative mid-sized 8x30mm Wildlife Pro CF, which features the benefits of Steiner’s renowned automatic focus system, as well as their new Ultra-Sharp focusing system for making critical refinements if needed.
There are times when the ability to focus on an object from a very short distance is critical, such as when observing wildflowers, butterflies, or birdwatching in a thickly forested area where the line of sight is limited to a few yards off the trail. Traditional Porros, due to their widely spaced objective lenses, cannot allow very close distance observation without the image being divided. However, by flipping around the Porro prism assemblies inside the binoculars, designers have been able to create inverted Porro prism models that, thanks to their objective lenses being even closer together than in roof prism models, can provide astonishingly close focusing abilities.
Among the best of these ultra-close focusing binoculars is the Pentax Papilio. Available in 6.5x21mm and 8.5 x21mm designs, the Papilio incorporates a revolutionary convergent lens design that allows focusing on objects from as close as 18 inches. In addition to the remarkable Papilio, Pentax also offers a wide selection of other inverted Porro designs, such as the UCF R and UCF WP series models. While not capable of providing the close viewing opportunities of the Papilio, they make good travel, event, and pocket binoculars thanks to their compact size.
Another reliable inverted Porro prism binocular designed with a relatively close focusing distance is the Nikon Travelite EX. Available in 9x25mm, 10x25mm, and 12x25mm models, the Travelite EX binoculars offer a small, easy-to-carry binocular that, thanks to their higher magnification levels relative to their close focus distance capabilities, can also be used for short distance observation of butterflies, birds, or perhaps even museum displays. An interesting recent addition to the Nikon line modeled on the Travelite EX series is the Nikon Ecobin, a 10x25mm inverted Porro prism binocular that uses lead-free glass, non-chloride rubber, and no harmful inks or dyes in its construction, making it one of the most green binoculars ever brought to market.
Before dismissing the idea of a Porro prism model as your next pair of binoculars, consider whether it what it can provide would be applicable to your intended use. Would an enhanced three-dimensional image make what you are viewing in the field easier to locate or observe? Might you need an ultra-close focusing distance in order to get a better look at insects or plants? While the answers to these questions might lead you to choose a model that isn’t widely popular, the basic rule of binocular choice should always be observed: the best binoculars for you are the ones that best match your ability to use them, and what you intend to do with them.
This article was written for The Binocular Site by John E. Riutta. To learn more about John please see his full biography.