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Basic Binocular Knowledge


What to know when purchasing and caring for your optics
by Jon Friedman


Birders, whether the backyard or in-the-field- variety, need only two essential items as the basic tools for learning about bird identification and behavior - a good field guide and reliable binoculars. (The best field guide for our area is the new Birds of Southeastern Arizona by Richard Taylor and has established itself as our best selling identification guide since its publication late last year). With just these two things, you can teach yourself all you want to know about the birds of our specific region, or anywhere for that matter.

Binoculars allow you to see images that appear many times closer than what the eye actually sees. This magnified view provides an in-depth image composed of detail and high resolution. Such a bright and color-accurate view allows you to observe and distinguish the most important details, even the smallest ones that the naked eye would not discern. It aids and encourages your knowledge and understanding of what you are seeing. It represents the difference between casual looking and critical vision.

Design Types

Manufactured in many countries, high quality binoculars are readily available. Particular types of binoculars are designed for different purposes. Some are designed specifically for birding and nature viewing (butterflies, insects, bats, and landscapes). Others have features designed for hunting, surveying and other long-range needs. Marine binoculars, designed for being on open waters, are salt-corrosion proof and made to protect eyes from the harsh glare and reflections from the water surface. Other design types are specifically for astronomical use, for the theater and opera, and for sports viewing. Compact models are designed for small size and weight. The larger the lens, the better the image is as a general rule. Most people balance out the features that are most important to them.

Vortex Diamondback 8x42


Evaluating binoculars for your personal use is important. A good pair of optics includes the following features:

  • A bright and detailed image under twilight (lowlight) conditions
  • An ergonomic design that fits and feels comfortable in your hand
  • A protective non-slip surface for a secure grip
  • Easy to use
  • Close focusing
  • As lightweight as possible
  • Wide field of view
  • Color accuracy
  • A good fit for your facial features and interpupillary distance
  • Affordable
  • Manufacturer's guarantee

Understanding how your binoculars function and perform enables you to make well-informed decisions. Some important technical considerations should be understood, as well. The following information provides a simple explanation of some of the basic features inherent in all binoculars.


8 x 42? What do the numbers stand for?

  • The first number refers to the power of magnification (how much closer the image appears compared to the naked eye).
  • The second number is the size of the larger objective lens measured in millimeters.

8 x or "eight power" means the image you are seeing appears eight times closer than seen with naked eye. More power of magnification is not necessarily better. As magnification increases, brightness and clarity may diminish, and the field of view is usually more restricted.

"42" means the width (diameter) of the objective lens in millimeters. There are two sets of lenses on each binocular. The larger lenses are called the objective lenses and the smaller lenses, the eyepieces, are called the ocular lenses. Generally speaking, the larger the objective lenses the brighter the image. In the same sense, the larger or smaller the objective lenses, the lighter/heavier and larger/smaller the binocular is.

Exit Pupil

Exit pupil is the term that describes how the magnification and the size of the objective lens work together and is a general indicator of how bright the image is. Simply, the exit pupil is the point at which all the light rays, after going through the lenses and prisms gather to form a magnified, circular image. By holding the binoculars at arm's length and looking into the ocular lenses, you will be able to see the bright circular exit pupil. For example, take a pair of 8 x 42 binoculars. Dividing the size of the objective lens (42 millimeters) by the power of magnification (8), you arrive at an exit pupil of 5.2 millimeters, which, for most birding needs, is ideal.

Most birders find using binoculars within the seven to ten power ranges fulfills their birding needs. Eight power is the most useful for all around birding needs. Exit pupils for birding needs begins at 4 mm and can exceed 6. A 9 x 63 has an exit pupil of 7! Exit pupils of 4 to five are perfectly acceptable for birding needs, but 5 or better are excellent for almost any birding needs.

Type of Construction

Porro or roof prism design? All binoculars are designed and constructed either as porro or roof prism design.

They can easily be distinguished at a glance. In porro prism designs, the ocular and objective lenses are offset while in a roof prism design the lenses are in-line. Porro prisms take the shape of the letter "M" ("W" if held upside down) while roof prisms look like the letter "H".

Neither is necessarily better than the other, at least that's been the case until more recent years. It was a matter of personal preference. If someone became familiar with either design type, it was more likely they would continue to use that type in the future. However, in more recent years, the roof prism designs have definitely become more popular, for a variety of reason. It used to be that porro prism designs were thought to give a sharper and brighter image with the primary drawbacks being that the binoculars were usually heavier in weight and larger in size. Due to major technological advances in the optical sciences, roof prism designs are now considered to be the preferential type - for a number of reasons.

  • Roof prism designs are smaller and more streamlined, more ergonomic (fits the hand better and feels better in the hand), lighter in weight, and definitely more rugged to hold up to rougher in the field handling.
  • Typically, roof prism designs will have much closer focusing ability than porros.
  • Porro prism designs are close focusing up to about 10 feet while roof prism models can close focus to as short a distance as 4 feet.
  • There are less internal parts in roof prism designs so less to go wrong if they are dropped or mishandled. Roof prism designs require more precise and compact tolerances in their manufacture. Their complex prism configuration also means a larger price tag than porro prism models of the same quality.

However, with today's technological advances, these differences are somewhat blurred. So, choosing a pair of quality binoculars can be enhanced by the basic knowledge the customer has in advance.

Eye Relief

Eye relief refers to the distance images are projected from the ocular lens to their focal point. The eye relief of a binocular can vary from 5mm to as much as 23 mm. Eye relief is important to eyeglass wearers in particular, as they need a minimum of 14-15 mm of eye relief to able to use the binoculars most effectively. The eyepieces of the ocular lenses should have either fold-down rubber eyecups or the more modern retractable (twist-up) style of eyecups for accommodating the user whether they wear eyeglasses or not. In short, eye relief allows for the proper distance our eyes need in order to stereoscopically focus as our naked eye does.

Field of View

Field of view is the size of the area you can see with a pair of binoculars. This measurement is the width of the image, measured in feet, at a distance of 1,000 yards from the viewer, as in "342 feet at 1,000 yards". Sometimes this measurement is expressed in degrees. A single degree is equal to about 57 feet. Therefore, a field of view of 342' is equal to a field of view of 6 degrees. Birders prefer a wide field of view to accommodate tracking moving birds and finding birds in the distance more easily. The larger the power of magnification, the smaller the field of view. So birders will prefer an 8 x 42 because it will offer a wider field of view, close focusing, lightweight, color accurate and be very bright. Hunters, on the other hand, will prefer a 10 or 12 power (usually requiring tripod mounting), which has a narrower field of view of about 275 feet, and a heavier optic with a less bright image.

Close Focusing

Close focusing refers to the minimum distance that the binoculars will focus to. The close focusing of any particular model depends on its optical design and, to a small extent, the viewer's own eye characteristics. For many birders and butterfly watchers, it is more desirable for binoculars to close focus at least 10-12 feet, while more technologically advanced models will close focus at five feet or less. Interpupillary distance, IPD, is the measurement between the center of the viewers eye pupils, expressed in millimeters. For many people, IPD is not a major consideration when purchasing a new pair of binoculars. But, for people with either narrow-set or wide-set eyes it can become a factor. Not all binoculars work equally well for all people, depending upon individual facial features.


Brightness or twilight factor is a measurement used to determine viewing efficiency and image detail in low light conditions. Different manufacturers use different methods or scales for this measurement, but generally the higher the number the brighter the image will be.

Lens Coatings

Lens coatings are also very important to consider. Factory applied lens coatings are extremely thin (measured in millionths-of-an-inch) and very hard. Each coating performs a particular function. The coatings can increase brightness, further assist detail and resolution to the edge of the image, block our harmful UV rays, prevent eye fatigue, and provide excellent and accurate color. Inexpensive optics may not have any lens coatings at all and are therefore the least quality.

  • "Coated" optics simply means that at least one of seven coating is applied.
  • "Fully coated" means the manufacturer has applied all the coating they use to the external, air-to-glass surfaces.
  • "Fully multi-coated" lenses means the maximum number of coatings are applied to both the external air-to-glass surfaces and the internal lens surfaces, as well.
  • "Super multi-coating" means the internal prisms, in addition to all the other coatings, have a phase-correcting coating on the internal prisms which give the viewer the brightest image possible.

Top of the line binoculars are super multi-coated, indicating the highest quality materials and workmanship, and contributing to their necessarily higher cost.

Care and Feeding of Binoculars

Prior to Purchase

Do your homework before purchasing new birding optics and obtain answers to any and all questions you have before deciding what to buy. Treat your purchase as a once in a lifetime opportunity. We always advise customers to buy the best model they can afford because if they are treated with respect and care there's no reason for the binoculars not to last a lifetime. We advise that you try out several pairs, taking your time, and don't buy anything until and unless you are satisfied that you have bought the best equipment in your price range. We sell Vortex optics exclusively. While we were authorized dealers in the past for Swift, Nikon, Pentax, and Audubon brands, we prefer the lower cost Vortex line as they offer all the bells and whistles and the latest technology that the more costly brands offer - but at a greatly reduced price. And, the unlimited unconditional lifetime guarantee that Vortex offers is the best in the business. Once you have made your purchase, proper care will provide you a lifetime of viewing pleasure.

Body Construction

The world of birding optics has changed dramatically in recent years. Technology is advancing optical science at a rapid pace. Many newer technological advances are incorporated into today's standard binoculars and spotting scopes. Optics are much lighter, brighter, more ergonomically designed, and give the buyer a better value for the money spent. Instead of the heavier aluminum bodies of yesteryear, many current models feature lightweight housings that give more strength and are lighter than aluminum. Today's better optics are constructed of rugged and durable magnesium, titanium, and/or polycarbonate.

Proper Care - Cleaning the Lenses

Unfortunately, when someone buys binoculars they are rarely advised on how to properly care for them. This is a serious omission, as proper care will lead to many more years of excellent use. Once you have bought a pair of binoculars, the first thing to do is learn about them and their care. The lenses are the heart of any optical instrument. Protecting them is the most important thing you can do for your optics. Many people make the very common mistake of cleaning them too often and/or using the wrong cleaning methods. These common mistakes can do serious damage to the fragile lens coating and drastically reduce the performance and lifespan of the lenses.

Micro-fiber cleaning cloths

Most optics may come with a felt-like piece of cloth that the manufacturer provides. We tell our customers to use it to polish their silverware or Micro Fiber Cloth their shoes. While these types of cloths were predominately used in the past, today's birders have better choices of materials from which to select. We have seen all sorts of materials used to clean lenses - scarves, mittens, shirttails, newspaper, Kleenex tissue, toilet paper, paper towels, etc. Scratched lenses are the result. We recommend using a micro-fiber cloth. These usually cost only a few dollars, can be washed when necessary, and will last a lifetime. A chamois cloth, cut into small and manageable size, will also work well. Avoid short-fiber and paper cloths. Using rubbing alcohol, seawater, or spit to get your lenses clean is another no-no. Unless you have fully waterproof, nitrogen or argon-purged optics -avoid using water or liquid lens cleaners.

Protecting Lens Coatings

Cleaning your optics too often or putting excess pressure on the lenses will wear out the coatings on your lenses. Lens coatings are integral to the quality of the lenses and images you see. Fully multi-coated lenses and phase-coated prisms control the total image quality; resolution and detail of the image, brightness, filter out the harmful ultraviolet A and B rays of sunlight, prevent eye muscle strain, reduce glare to the absolute minimum, and provide color accuracy. Only clean your lenses when it is obviously necessary.

When holding your binoculars up to your eyes and finding your vision obscured, it is definitely time to clean them. Dust is the most common culprit. If you leave your optics out of their case, without the lens covers on, you will notice that dust settles on the lenses in no time. A dry blast of air will usually eliminate 95% of any dust. Physical cleaning is not necessary in this situation.

On the other hand, one of the most difficult materials to remove is skin oil. If you accidentally touch a lens, you will leave a little grease spot or oily smear. If allowed to remain, this affects your lenses in a negative way and if not cleaned off while fresh will prove much harder to effectively clean at a later date. Most folks, without proper instruction, simply take a clean cloth, apply gentle downward pressure of the lens, and, with a circular motion, rub on the oily spot. It gets thinner and thinner as the oil is absorbed into the cloth. While this method seems to work, repeated cleanings over time using this technique actually erodes the coatings as well ridding the lens of the oily spot. A better and less abrasive method is to use a clean micro-fiber cloth wrapped around a single finger and approach the spot from the rim of the lens to the center of the lens while turning, or rolling, the finger up and away from the lens. This can be repeated from more than one angle until the oil spot is removed. This "lifting off" method avoids using any downward pressure, protects the lens coatings and eliminates the difficult spot.

Do Nots

Do not eat or drink while wearing your binos around you neck. Food or drink accidentally spilled on these is one of the most common mistakes. Never immerse them in water or drop them in sand. Do not attempt to lubricate hinges or focusing wheels. Never try to clean the interior surfaces of any lens. Tampering with the lenses or housing in an attempt to clean any interior lens surface, mirror, or prism will void any warranties that the manufacturer offers. It is always best to return them to the manufacturer and have their skilled factory technicians do the work professionally. This is always the best solution for any internal cleaning that may be necessary. The necessity for this type of cleaning is usually a result of mishandling, accidents or a manufacturer's defect in workmanship or materials. Rather than return them to the manufacturer yourself, have authorized dealer return them for you. By returning them to the authorized dealer you will usually have them returned quicker and avoid any unnecessary costs.

If you follow these few common sense tips in caring for your binoculars, they will provide you with many years of good viewing and save you the expense of having to buy new optical equipment. If you have any questions about the care and cleaning of your optics, bring them into the store for a demonstration of proper cleaning and get the answers you need about your optics. The Wild Bird Store is the authorized dealer for Vortex Optics in Southern Arizona. We firmly believe that Vortex optics offers our customers the very best quality optics for the very least price.

The more knowledgeable you are about binoculars, the better your selection will fit your personal criteria and budget. If you are ready to up-grade the pair you have been using for the past 5, 10 or twenty years - come into our store for a demonstration. We are ready to help you find just the right pair, one that fits your needs and budget.

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