Common questions about binoculars:

Binoculars can open your eyes to animals and scenery you may have never noticed otherwise. Unfortunately, the decision to buy a binocular can be very difficult when presented with the huge array of possible choices available. Our aim in answering these commonly asked questions is to give you the best possible advice, so you can choose binoculars that really do fit your needs.

What do the numbers in a binocular name mean?

The numbers describe the configuration of a binocular. Let’s take 8×42 binoculars as an example. The first number (8) refers to the magnification, or, how many times closer an object will appear when looking through the binocular compared to looking through eyes only. The second number (42) indicates the diameter of the objective lens (the light-gathering lens) of the binocular and is measured in millimeters.

Note: The diameter of the objective lens is directly related to the size of the binocular — the larger the objective lens, the larger (and often brighter) the binocular.

How much magnification do I need?

You will want a binocular with magnification appropriate to how you plan to use your binoculars. So, think about the demands of your hobby before you select a pair of binoculars. How much detail do you need to see? Will you use your binoculars to view things close up or far away? Will you be able to prop your arms on something for support while using your binoculars? Do I want a small binocular that I can take anywhere?
The most popular binoculars are those with lower magnifications (as with 8x binoculars). These binoculars have wider fields of view and are easier to hold steady than the binoculars with higher magnifications. A wider field of view is important when trying to follow fast-moving action like game on the move, warblers on the wing, or athletes at a fast-pace sporting event.
Higher magnifications (as with 10x binoculars) will give you more detail, but are more difficult to hold steady than binoculars with lower magnification. Binoculars with higher magnification also have narrower fields of view. While many people choose to use 10x binoculars, they are deciding that image size and detail is of greater importance to them.

Note: The diameter of the objective lens is directly related to the size of the binocular — the larger the objective lens, the larger (and relatively brighter) the binocular.

What are Porro prism binoculars and roof prism binoculars?

Roof prism, Porro prism (and reversed Porro prism) binoculars each use image-erecting prisms to provide correctly oriented images. You will recognize the Porro prism as a “traditional” binocular design with offset prisms while the roof prism binocular has parallel barrels and a more streamlined appearance. The size, design and quality of the prisms used in the binocular design affect what you see and how clearly an image appears across your viewing area. You may prefer one binocular over another because of the design.

Porro Prism Binoculars
Porro Prism models will deliver the best optics for the dollar, but lack the durability and compact styling of roof prism models.

Reversed Porro Prism Binoculars
This design allows for a compact binocular that easily fits in your hand.

Roof Prism Binoculars
Roof Prism models will deliver more durability but you will need to spend more money to get good optical quality in this design.

What size binocular do I really need?

Binoculars can be classified as either full-size or compact binoculars. The size of the objective lens affect how large or small a binocular is.

Full-Size Binoculars
Full-size binoculars (like an 8×42 binocular) offer the greatest light gathering ability and over-all optical performance.

Compact Binoculars
Compact binoculars (like an 8×25 binocular) are more portable than full-size models and work well during daylight hours, but not during twilight or in deep shadows.

What is the field of view in a binocular?

When you look through your binoculars, the widest dimension you can see is known as the field of view. Some binoculars will feature unique lenses to provide a “wide field” that is greater than normally seen through binoculars of the same magnification.
A wide-field binocular is desirable for observing at close quarters in deep woods or picking up anything that is moving quickly across your viewing area.

Note: The field of view decreases as magnification increases, so select a binocular with lower magnification if a wide field of view is important to you (or select a binocular with a wide field feature).

How much should I spend on a binocular?

Buy the best quality binoculars you can afford. This will allow you to spend the greatest possible time with your binoculars pressed to your eyes. You’ve probably heard the old statement about fishing that says “to catch fish, you need to have your line in the water”. Well, you won’t see more detail unless that binocular is at your eyes. Poor quality binoculars usually spend a lot of time just dangling from the neck.

Note: Poor quality binoculars also lead to eyestrain, headaches, and are not very fun to use for any length of time. In the long run, you’ll probably save money if you initially buy better quality binoculars and avoid trading up later. In the meantime, you’ll see so much more–so much better!

How does optical glass enhance binocular image quality?

The optical glass used in a binocular affects the image quality you see through your binocular. The objective lens of a binocular refracts or “bends” light, attempting to direct that light to a certain focal point at some distance behind the lens of the binocular. Each color or frequency within the visible light spectrum has its own characteristics. As the light passes through the lens, each color bends at a slightly different angle than any other color and, therefore, the various colors focus at different points. The result is a lack of image sharpness and poor color quality.
Certain binoculars incorporate special cost- and labor-intensive glass in the objective lens to correct for chromatic aberration and color fringing. You’ll hear about ED (extra low dispersion) and HD (high definition) glass. When used in binoculars, these glasses provide highly resolved images with enhanced brilliance and exact “true-to-life” color. Even the finest structures will show up well in high contrast and clarity.

Note: For the more serious observer, the enhanced image quality will be worth the extra cost.

What about light transmission and optical coatings?

Light transmission describes the percentage of available light that passes through your binoculars. The amount of light can be increased with the use of anti-reflective coatings on binocular lenses. When an anti-reflective coating is applied to the glass surface of a binocular it increases the amount of light that reaches your eye.
The more complete and complex the coating scheme is, the brighter and sharper the binocular image will be. Better coatings on a binocular allow more light to reach your eye, improving brightness and overall binocular performance.

What if I wear eyeglasses while using my binoculars?

Many models of binoculars will allow you to view in comfort while wearing eyeglasses or sunglasses. These binoculars have been designed to provide you with longer eye relief.
Eye relief refers to the distance images are projected from the ocular lens to their focal point and can vary from 5mm to 23mm. If you want to use your binoculars with eyeglasses or sunglasses, look for binoculars that offer at least 15mm of eye relief.

Note: Without proper eye relief, the eyeglass wearer will not see the full field of view.

Binocular Eyecups
When using your binoculars, be sure you understand the correct use of the eyecups. All binoculars will allow you to adjust the eyecups. There may be a rubber eyecup that can be folded down or a newer type that either twists or slides up and down on the eyepiece of the binocular.
If you’re an eyeglass wearer using binoculars, you always want to be sure you have the binocular eyecup rolled back (placed in the down position) while using your binoculars. This will allow you to see the widest possible field of view through your binoculars. Conversely, if you don’t wear glasses, you’ll enjoy your binoculars more if you leave these eyecups fully extended (placed in the up position). This will allow the binocular eyecups to just comfortably touch your face and help block out lateral light.

Do my binoculars need to be waterproof?

Binoculars are used outside in all kinds of weather. If you expect that you will be using your binoculars in rainy, wet weather (or if you use them around water), consider a pair of waterproof binoculars. You’ll pay a bit more for this feature, but you’ll also be able to use these binoculars without fear of repairing or replacing them just because they got wet.
Waterproof binoculars are literally submersible and any water damage would be handled under the manufacturer’s warranty for the binocular.

Note: If waterproofing isn’t necessary for your hobby, then you can find good quality binoculars at considerably lower prices.

What makes a binocular appear brighter?

Brightness in your binoculars can be affected by several things. Contrary to what many binocular users believe, big objective lenses are not the only factor to consider. The quality of glass used in binocular prism and lens construction along with the quality of the lens coatings on the binocular contribute more to brightness than do big objectives.
Lens coatings are thin layers of chemicals applied to the glass surfaces of the binoculars. These coatings improve light transmission through the binocular. Coating quality is affected by the number of coatings and whether all binocular lenses, inside and out, are coated with several layers of coatings.
Ideally, you should purchase binoculars that are fully multi-coated, which means both sides of every binocular lens are coated with at least several layers of the anti-reflective chemical. High quality lens coatings will appear as fairly light, subtle shades of blue, green or violet.
A word of caution, beware of a binocular using heavily colored lenses. These lenses will only cut down on light transmission through the binocular.

Note: If all other things are equal in two binoculars, a binocular with a larger objective lens will yield a brighter image, but at the cost of greater size and weight. Higher quality binoculars with a smaller lens may very well be brighter than less expensive binoculars with larger objective lenses.

How do I clean my binoculars?

Use common sense in the care and maintenance of your binoculars. Always attempt to blow off any visible dust or dirt from the binocular lenses before brushing or rubbing anything on the glass of your binocular. Next, use a lens cleaning tool like a Lens Pen or lens cleaning tissue to gently wipe off any remaining marks or spots from the lens of the binocular.
Remove stubborn things like dried water spots from the lens of a binocular by lightly fogging the binocular lens with your breath. If your waterproof binoculars are badly soiled, you can even clean them by placing them under lightly running water (to minimize any possible damage).

Note: Don’t use your shirt tail or pocket tissue to clean your binoculars. These fibers may contain material which will scratch the coatings on the binocular lenses.

Keep binocular eyecups and focus mechanisms free of dirt and oil. An occasional wipe with a vinyl or rubber preservative like ArmorAll will extend the life of rubber eyecups on the binocular. Check your neckstrap and its attachments for wear or slippage, too. You don’t want your bino to come flying off and hit the ground!

What about the light gathering ability of my binocular?

Your eye is uniquely designed to gather more or less light as conditions change, but your binoculars are not. Since the light gathering ability of a binocular is fixed, it is important to select a model that best meets your eye’s need for light as viewing conditions grow darker.
The exit pupil is the magnified image in the eyepiece as it leaves the binocular to enter your eye. It is an indicator of how well you will see an image through your binoculars on a bright day, at twilight or at night. Almost all binoculars gather more light than is needed by your eye for viewing in bright conditions. For the best viewing at twilight, you’ll want a binocular with a minimum exit pupil of 4mm.

Determining the Exit Pupil
To determine the exit pupil of a binocular, simply divide the objective lens by the magnification of the binocular. If we use an 8×42 binocular as an example, the 42mm objective lens divided by the magnification of 8x gives us an exit pupil of 5.25mm. This tells us that an 8×42 binocular is a good configuration for low-light viewing.
In contrast, if we look at an 8×25 binocular, we find that 25mm divided by 8x leaves us an exit pupil of only 3.12mm. This means that an 8×25 binocular will give sufficient brightness for daytime viewing, but the image will not be as bright as you would like at dawn and dusk.

How do I properly set the diopter on my binocular?

  • All better quality binoculars will allow you to separately adjust the focus on one eye (usually the right eye) with a diopter adjuster. This is done to compensate for differences between your eyes so you see the clearest image possible through your binoculars.
    To make the diopter adjustment, pick a distinctive object to focus on that is beyond the close focus distance of the binocular, but not too far out in the distance. Initially, come to a sharp focus on the object using the center focus of the binocular (with the diopter-corrected eyepiece side blocked off or that eye closed).
    Once you’ve got the first eye focused as sharply as possible, close it and, then, open your other eye (the one on the diopter-corrected side). Carefully, without moving the center focus of the binocular, see if you can improve the image sharpness through this eye by slowly moving the diopter adjustment back and forth. Once you’ve found the setting which gives you the sharpest image, note it and leave it there — you’re done. From this point on, you simply use the center focus to adjust both eyes while viewing.

    Note: You should properly set the diopter adjuster when you first use your binocular (make note of your setting). Check the diopter setting on your binocular every so often to be sure your eyes haven’t changed. Always check it whenever you share your binocular with someone else.