Be the Expert: Prism Optics to Focus Sales

Prism Optics

Focusing on Binocular Terms

Binocular terms can be confusing. Be the Expert turns its eye on prism optics to educate your sales team. This, in turn, enables them to work with your customers, so together they can decide which pair of binoculars will work best for their needs.

What do the numbers mean?

Besides whatever marketing name a binocular has, the first key piece of information is those “X” numbers—like 10×50 or 2x 24mm or 10-30×60. Although the use of spaces or mm can vary, it all means the same thing.

  1. The first number is the magnification. So 10x means anything seen through the binocular will appear 10 times closer than the naked eye. The dash, like in 10-30x, means it’s a zoom lens.
  2. The second number, whether it uses the mm or not, is the diameter of the objective lens. As standard in optics, that measurement is in millimeters. The objective is the outside lens, the piece of glass that faces the “object” viewed. The larger the number, the larger the lens.
  • Is a large lens a good thing? It all depends. The larger the lens, the more light passes through it, making the image clearer, sharper and brighter. It also means the lens is heavier, the housing both larger and heavier and the cost more expensive.

Prisms are NOT where naughty light rays go!

There are two basic kinds of prisms used in binoculars—Porro and roof. As you can see from the illustration from, the shape of a binocular indicates the prism. Porro prism binoculars are wider and shallower while roof prism binoculars are skinnier and longer.

Porro prisms were invented by Ignazio Porro, so the P is always capitalized. In a Porro, the light path is folded and requires four reflections.

The roof design, also known as an AK or dach (German for roof) prism, brings the light path in a straight line and requires six reflections off the roof surfaces in the process.

Each has its strengths and weaknesses:

  • Porros’ basic optics are better than roof and cheaper to manufacture. But they are bigger, bulkier and harder to weatherproof.
  • Roof can be optically better than Porros, but to get there, they require more expensive coatings such as phase correcting. They are lighter in weight, more compact and easier to weatherproof.

Eye on glass

The kind of glass the prisms are made of is equally important. The two most common terms you’ll see are BaK-4 and BK-7.

  1. BaK-4 is shorthand for Barium Crown Glass, although some manufacturers use phosphate crown glass. It transmits light better and is considered the best type of material,
  2. BK-7 is made from borosilicate glass. It is more widely used due to its more affordable price. It is optical glass, so it’s good. But it’s not on the same level as BaK-4.

Lens coatings

To prevent glare, remove unwanted reflections and improve color, the binocular lenses are coated. As you might guess by now, the kind of coating used affects both purpose and price. Note the terms do not indicate how many times a coating might be applied.

  1. Fully coated. At least one lens is coated.
  2. Multi-coated. This can mean multiple surfaces are coated or that multiple layers are on each surface. So it’s a bit ambiguous.
  3. Fully multi-coated. All surfaces of the lenses have multiple layers of coatings.
  4. Broadband fully multi-coated. Light has wavelengths, so these coatings are effective across a wide spectrum.

The math of binocular optics

Other important numbers associated with binoculars are Field of View, Angle of View, Minimum Focus and Exit Pupil.

  • Field of view (FOV) is the width of the visible scenery at 1,000 yards or 1,000 meters. The more magnification there is, the narrower the field of view.
  • Angle of View (AoV). A 6.3° angle of view means a user is seeing 6.3° of the entire 360° surroundings. AAoV is the Apparent Angle of View. Figure this by multiplying the AoV by the binocular’s magnification number. This gives the angle of the magnified field when looking through the binoculars. Anything more than 60° is considered wide angle.
  • FOV and AAoV are mathematically related as well. 1° = 52.5 feet at 1,000 yards. A 10x 42 binocular with an FOV of 360 ft has an AoV of 6.9° (360 divided by 52.5) and an AAoV of 69° (6.9 x 10).
  • Minimum focus. Since typically binoculars are for distance viewing only, any customer looking for a short focal point should look at a large objective lens and a magnification of 8x or less.
  • Exit pupil is determined by the basic optics of the binoculars. It is not affected by the glass or coatings. Exit pupil is the circle of light visible at the eyepiece. You can calculate it by taking the objective number and dividing it by the magnification number. Large exit pupil numbers, like 5mm to 7mm, are better for low light conditions.

Construction details

Beyond these basics, there are nuances. Each appeals to a different kind of user.

  • Eco-Glass. Lenses made without lead or arsenic so they are considered eco-friendly.
  • ED Glass. Extra-low Dispersion glass in lenses provides high-definition images as it has almost no distortion and doesn’t bend the light waves.
  • Fogproof. Instead of regular air inside of the housing, there’s nitrogen, argon or both. These inert gases prevent fogging. All fogproof binoculars are waterproof.
  • Center focus, individual focus and focus-free. The focus wheel is on the bridge or center hinge. Some models allow each barrel to focus independently. Focus-free is just that. Everything is preset at the factory. 40 yards tends to be the minimum range.
  • Diopter adjustment. This allows one or more eyepieces to be fine-tined to match the eyeglass prescriptions. Some models have this adjustment as a part of the center focus wheel. Those with more specialized prescriptions may find this inadequate.
  • Eye relief. This means binoculars can be held away from the eye and still show the full field of view. This is very useful for those who wear glasses and find the diopter adjustment insufficient. 15mm is generally considered the minimum distance for those who wear glasses.
  • Inter-Pupil Distance (IPD). The distance between each eye varies from person to person. So adjusting the hinge of the binoculars aligns each eyepiece.
  • Eyecups. There are various kinds. All add comfort to extend viewing time. Fold-down are helpful to those with eyeglasses.
  • Image stabilized. Just like camera lenses can have image stabilization, so can binoculars. This helps if on a boat or in an airplane.

Classy chassis

A wide variety of materials are used to house the optics:

  • Aluminum is lightweight, inexpensive and corrosion-resistant.
  • Magnesium is lighter, yet it’s very strong and corrosion-resistant, as well.
  • Polycarbonate shares the characteristics of above, plus it’s temperature-resistant.
  • Rubber or other synthetic materials can coat the chassis. This provides armoring to help protect the binoculars from scratches. Plus it makes them more comfortable to hold.

Matching the right binoculars to the customer

So, with all the variations out there, how do you help someone who comes into your store? There are two overarching principles, each with a trade-off:

  1. Environmental conditions. Porro prisms are very hard to waterproof. Most manufacturers don’t try. Roof prisms, when specially treated, work well in wet conditions.
  2. Price. Porros are inherently less expensive and have a better image. Roof prisms can cost less too, but bringing the image up to snuff requires expensive coatings.

So, ask your prospective buyer questions. You need to find out the application, type of weather conditions they’ll use them in and the preferred price range.

  • Hiking, sports and outdoor activities. These call for compact binoculars with 7x to 10x magnification. Weather conditions and price range are factors in choosing Porro or roof.
  • Hunting. 7x to 10x works well, too. But for long-range applications, 12x to 16x might be best. But that magnification requires a tripod. Waterproof roof prisms are recommended unless the hunter is fair-weather only.
  • Bird watching. 8x 42mm tends to be standard. For more detail, 10x or 12x with a 42mm to 50mm objective lens. Choose eye relief. Consider weather conditions and price range in choosing Porro or roof.
  • Opera/concert/theater. There are special binoculars called opera glasses. They are small binoculars with a magnification of 3x or 4x. Other kinds of binoculars used for this same purpose are 4x 30mm, 7x 18mm and 8x 25mm.
  • Boating/marine. Start with 7x and maybe go up to 10x magnification with a 42mm to 50mm objective lens. This usage calls for waterproofed and armored roof prisms.

Prism Optics focus the sale

Understanding prism optics and other basic terms is important for your sales staff. Plus many of these same terms apply to monoculars and spotting scopes. Once they know the ins and outs of prism optics, it will be much easier for them to match each customer to just the right piece of glass.

And to ensure you have a wide variety of binoculars in stock, be sure to check out our wide selection at!


1 Comment

  1. Robert Johnson Reply

    This is a great article. One additional use is Astronomy. Binoculars can be used to see deep space objects, the moon and the planets. When looking at a planet like Jupiter, you can make out the four Galilean moons, though you won’t see much detail on the planet itself as it’s too bright.
    For handheld astronomy use, it’s recommended to use 50mm objectives and lower magnifications. Binoculars with an objective larger than 50mm will be more difficult to hold steady on a target, because of their size, and will likely require a tripod or require the user to be in a position where they can keep the binoculars steady (e.g. laying down or reclined in a chair). Higher magnifications make objects more difficult to keep steady when handheld. For astronomy, keep the magnification in the range of 7x-10x unless it’s mounted on a tripod or a parallelogram mount. There’s not much of a difference between 7x and 10x for astronomy and the 7x will be easier to keep steady but 10×50 is the most popular size. Porro prisms are generally preferred over Roof prisms, though I’ve used both with success. If one can afford them, image stabilized binoculars are great for astronomy because they reduce the natural tendency to move while looking up. In general, it’s recommended to avoid zoom binoculars for astronomy.
    A well-corrected objective is important as well. Cheaper binoculars will likely show distortions at the edge of the field of view which may not be obvious during the daytime. Stars may look fuzzy or elongated. The wider the field of view, the more difficult it will be to get a clear distortion-free view.

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