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Designs


Quality and Design

blue bird If you could graph the quality/price ratio, you would find that quality increases proportionately with price until the very top of the line where price goes up sharply for that last little bit of features. High-quality optics have multi-element lenses with several coatings to optimize the visible light spectrum, very close tolerance manufacturing processes and the best optical supports, consistency, and alignments available. To get optimum performance, durability and consistency, there is no way around the expense to produce the best.

On the other hand, middle price range optics ($400-$700) in today’s market can be very fine instruments. The optics in this category are really making great strides. If price is a major consideration, several of the mid-range optics may not have quite as many elements (both objective and eye piece), features, and options but are still fine optics with only slight differences in viewing ability under most situations.

That said, especially in low-light conditions, you truly do get what you pay for in optics. Birders who are demanding of their optics will want the best they can afford. High-quality optics perform well in all conditions, deliver the sharpest image with the truest colors and have ergonomic designs that maximize comfort during use. Even with the best optics, there are many considerations and choices. There are also some trade-offs that will be determined by each individual's needs and wants. There is no ideal design or "star wars" pair of binoculars on the market, and the optics you choose is best determined by what you want from them, the conditions under which you will be using them and what you can afford.

Porro and Roof Prism Designs

In modern binoculars and spotting scopes there are two main designs. These are the porro and roof prism designs as shown in Figure #1 and Figure #2 below. The porro prism was the first of these two designs and consists of two mirrors mounted separately. The roof prism is a more modern design that consists of two prisms cemented together in a single interior mount. In both designs, light passes through objective lenses that cast it onto a set of mirrors that further flip, rotate and focus the image through an eyepiece and into our eyes. The final view is brought into sharp focus with the focusing knob that changes the distance between the mirrors and either the eyepiece or objective lens. There may be as many as 10 pieces of glass in each barrel that the light passes through before the final image reaches our eyes (typically a minimum of 6).

(Figure #1)

(Figure #2)

binocs9 binocs8

Porro Prism

Roof Prism

Quotes from Charles A. Bergman, Audubon, November 1981

Porro prism (see Figure #1 above)

"But how do binoculars present an upright image to the viewer? ...An Italian named M. Porro invented the first prismatic inverting system in the middle of the nineteenth century...Porro's system consists of two identical prisms in each barrel of the binocular, placed at right angles to each other. Each prism looks rather like one of Napoleon's hats, the bicorne. Like mirrors, each prism reflects and reverses the light off its steep sides. One prism reverts the image left to right and the other prism inverts the image top to bottom. With an acrobat's grace, the image somersaults through the binoculars, springing to our eyes, normal and erect, a natural perspective."

Roof prism (see Figure #2 above)

"Roof prisms were invented in the nineteenth century, in the optical workshop of Carl Zeiss at Jena, Germany. A young professor of physics at the University of Jena, Ernst Abbe, formulated the mathematical laws for the paths of light through microscopes. Together with the chemist Otto Schott, he also invented the first high-quality, reproducible optical glass. At age twenty-six, he became director of research for Zeiss. Using his own mathematics and glass—both historical contributions to the optical industry—Abbe created not only the microscopes of Louis Pasteur, but also a pentaprism, the "Abbe roof prism." Abbe's roof prism was the progenitor of a vast array of modern roof prisms. Inside, roof prisms juggle the light rays through an invisible labyrinth of angles, a convoluted path, a mathematical marvel. On the outside, a roof-prism binocular is elegant in its simplicity."

Practical differences

porro prism The differences in durability, image and optical quality in binoculars and scopes are most pronounced between lower and middle priced optics with less dramatic changes between middle- and high-priced optics. The two types of optic designs reach peaks in each of these factors at different price levels.

The only difference between the two designs in full-sized binoculars that does not change with price is their size and shape. Porro prisms are larger and more bulky than roof prisms. Most bird watchers are inclined to stick with the slim roof prism design although there are those with big hands that claim that the larger porro prisms are more comfortable for them (I have large hands and do not find this so).

Porro prism binoculars reach very good optic quality at about $250. Roof prisms usually do not reach an equivalent level of optical quality until about $400-$700. Roof prisms, because of the engineering required, are more expensive than porro prisms of equivalent optical quality. Modern roof prism binoculars tend to have two separate prisms that are cemented together, rather than two offset prisms of the porro prism design. Porro prism designs consequently tend to be much more susceptible to alignment problems if dropped. This can be very costly to fix. Thus, both weather resistance and durability are better in roof prism binoculars.

Roof prism designs are almost exclusively internal focusing and that makes them much easier to seal and keep waterproof. Even expensive waterproof porro prism designs typically are sealed with "o" rings and are not internal focusing. The mechanical movements of the eyepiece (or objective) moving back and forth, and wear on an "o" ring can cause problems over time. First is loss of the seal, allowing moisture and dust into the interior. Moisture and dust settling on the mirrors degrade the performance. Next, wearing of an "o" ring causes focusing friction. As the "o" ring degrades, the focus becomes progressively stiffer. The need for nitrogen (or other dry gas) fill and sealed optics to prevent internal fogging and condensation are discussed in the weatherproof section.

Reverse Porro Prism


(Figure #3) (Figure #4)

Not to confuse the previous statements about the size and shape being the only constants that do not change between porro and roof prism designs, we should mention reverse porro prism binoculars. In the case of compact binoculars there is also a reverse porro prism design. These are small, nice lightweight designs. The difference is shown at the left.

porro1 Regular Porro Prism porro2 Reverse Porro Prism

Compact binoculars

Although we do not recommend compact binoculars for primary use, they certainly do have their place. These are smaller optics that generally range from 20-32 mm in objective size and 6-10 power. The next article (Basics II - Designations and Considerations) addresses issues such as magnification, exit pupil, and objective diameter that are especially important considerations when choosing compact binoculars.

Compact binoculars are appropriate when full-size binoculars are too large to carry (such as while biking, backpacking...), as extra binoculars left in a car glove box, or maybe for small children. Especially with children, the choice of compacts deserves careful consideration since they may be more difficult to use because of a small exit pupil. Compact binoculars have the advantages of smaller size and reduced cost.

All issues considered in choosing full-size binoculars also hold true when choosing a pair of compact binoculars. Weight is less of a consideration but magnification becomes a greater consideration since small binoculars have a smaller objective lens. They can even be more difficult to hold steady and the smaller objective lenses transmit less light.

As you read the following article discussing the choices and options that make for a good pair of binoculars, the cautions and considerations above apply to this choice. The only unique choice is in the reverse porro prism. Reverse porro prism designs are generally confined to compact binoculars because as the objective diameters become larger the distance between the eyepieces becomes prohibitively great.>

Design Conclusions

  • Both porro and roof prism binoculars can deliver excellent optical quality.
  • Neither is inherently lighter or easier to use for any individual.
  • Optical quality comes at a cheaper price with porro prisms.
  • Slender design and durability is better associated with roof prisms.
  • Top of the line optics in either design are comparable in all aspects, including price.

Previous Article - Introduction | Next Article - Basics II

Learn About Optics


Day Optics

Designs - Quality, compacts, porro and roof prism designs for binoculars and scopes...
Designations and Considerations - Designation values, eye relief, weight & cups, exit pupil, and twilight factor...
Additional Consideration - Focusing, field of view, depth of field, weather proofing and nitrogen fill...
Optic Components & Image Quality - Lenses, mirrors, coatings, aberrations, distortions, and alignments...
Spotting Scopes - Construction, Objective lens, eyepieces, angled or straight, and focusing...
Tripods - Heads, legs, monopods, shoulder stocks, and window mounts...
Digiscoping - About, power, editing, considerations, cameras, techniques, and effects...
Care & Tricks - Holding techniques, cleaning, carrying, and protecting your optics...

Night Vision

Starlight Technology - NV Types, Starlight Technology defined, basic design and IR Illuminators...
Starlight Technology Night Vision Generations and Devices - Generation 1 to 4 - levels of NV technology, types of devices and their uses...
Use & Care - How to use, controls, and care for NV devices, extending capabilities...
Digital Night Vision and Thermal-Imaging - Digital NV and Thermal Imaging, how they work and compare to standard NV...

Buying Guide

Binoculars - All the factors to consider when buying binoculars.
Bins for kids - Special Considerations for children's binoculars.
Challenged - Special considerations with binoculars for the physically challenged.
Spotting Scopes - All the factors to consider when buying a spotting scope.
Tripods - Selecting the best tripod for your scope.