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Birding Optics

Binoculars: The facts you need to know in order to make the right choice of binoculars for your birding adventures Contains a non-technical explanation of how binoculars work.

This article was originally published in Bird Watcher's Digest in 1993. It was later reprinted by the American Birding Association and featured on

Binocular parts. Birding binoculars have three basic elements: the front lenses, the rear lenses, and the prisms. The front lenses are called the objective lenses. They gather the light and focus an image of the object inside the binoculars. The rear lenses are called the eyepieces. They enlarge this image and present it to the eye. The third element are the prisms, located between the objective lenses and the eyepieces. The prisms function as mirrors, reversing the image and turning it right side up. Without the prisms, the birds would appear upside down and backwards. The prisms also fold the light path so that the overall length of the binoculars is shortened.

What birders need? Birders demand a lot from their binoculars. Birding binoculars must be light enough to carry all day long and sturdy enough to survive years of heavy use. They must be easy to hold steady. They must resolve delicate details and reveal subtle colors with accuracy. They must focus quickly and up close and work well in dim light. They must be sealed from dust and moisture. And they must show the whole picture even for birders wearing eyeglasses.

What's best? To select the best binoculars, you must choose them personally. Only you can decide how much magnification you can hand hold steadily and how much weight is comfortable to carry. Only you can feel how binoculars fit in your hands and how well they work with your eyeglasses.

To make the best choice you also need a basic understanding of how binoculars work, so that you won't be confused by the technical data. Knowledge is the compass that lets you navigate the informational sea.

How binoculars work. Essentially, binoculars are just two telescopes mounted side-by-side, one for each eye. To understand binoculars, you need to understand how a telescope works. Here's an easy demonstration that you can try yourself. All you need are two ordinary magnifying glasses and a piece of tracing paper. Do this once, and you will understand forever how binoculars work.

Hold the tracing paper on the opposite side of the magnifying glass from a bright object, such as a light bulb. Move the paper back and forth. At a certain distance, an upside-down-and-backwards image of the light bulb will form on the paper. You can enlarge this image by examining it through another magnifying glass. You may be surprised to find that if you slide the tracing paper away, the image will remain, only brighter and clearer. You have just made a working telescope.

The magnifying glass nearest the object is called the objective lens; the one nearest your eye, the eyepiece. The objective lens and the eyepiece are two elements in all binoculars. Binoculars also have a third element, the erecting prisms.

In the telescope we just built, everything is upside down and backwards. That would be OK for looking at stars, but for watching birds or following the action at a football game we require a right-side-up picture. A terrestrial telescope has to flip the image, and that's what prisms do.

A prism is a solid piece of glass that functions as a mirror, but without a mirror's reflective backing. Light rays that have entered a prism cannot get out if they strike a surface at too great an angle. Instead, they reflect back, as if from a perfect mirror.

In the mid 19th century, an Italian named Porro designed a telescope with two prisms set at right angles to each other between the objective lens and the eyepiece. This arrangement not only erected and reversed the image, but also folded the light path, resulting in a shorter, more manageable instrument. In 1894, the Zeiss Optical Works created the first "Hunting Glasses," incorporating the Porro prism design, and modern prismatic binoculars were born.

All binoculars still have these three parts. An objective lens focuses an upside-down image. A set of prisms turns the image right side up. And an eyepiece magnifies it. Though modern eyepieces and objective lenses are each comprised of multiple elements, their basic functions remain unchanged.

Porro prism vs. roof prism Porro prism binoculars were standard until the 1960's, when the Zeiss and Leitz companies introduced roof prism binoculars, whose objective lenses were straight in line with the eyepieces. Roof prism binoculars were compact, light, and comfortable to hold. They made the offset, zig-zag shape of the Porro prism design look as old fashioned as propeller-driven aircraft. Roof prism binoculars appeared simpler than Porro prism binoculars. But inside, they had a more complex light path and required much greater optical precision in manufacturing. As a result, they cost more to make. The Porro prism design was simpler and more light efficient, and its images showed better contrast. Nevertheless, the roof prism design's appeal was so great that manufacturers went all out to perfect it.

They succeeded. Today, roof prisms dominate the top-end birding binocular market. Porro prism binoculars are not obsolete, however. Dollar for dollar, a Porro prism design will give better performance for the money, especially in medium or low priced binoculars.

Better Porro prisms binoculars are made from a high density glass, BAK-4. If you hold binoculars away from your eyes and up to the light, you can see the circular exit pupils in the eyepieces. The less expensive BK-7 prisms will have squared-off, non-circular exit pupils.

Coatings: through a glass darkly Each time light enters or leaves a piece of glass, about 5% is reflected back. Binoculars may have 16 air-to-glass surfaces, with light lost at every surface. In early binoculars, less than half the light got through to the eye. The rest bounced around inside the binoculars, making the image hazy and hard to see, like a movie in a theater with the lights on.

Bigger objective lenses can compensate for the lost light, but they result in heavier binoculars. In the 1940's, it was discovered that coating the glass with magnesium fluoride would let more light through. The original coating technology was a single layer, which reduced reflections to 1-1/2% per surface, instead of 5%. More recently, advanced multi-layered coatings have reduced reflections to as little as 0.25% per surface. Today, in the best binoculars, 95% of the light gets transmitted to the eye.

Coating technology depends on applying perfectly uniform thicknesses, a few millionths of an inch thin. A little too thick or thin, and the coatings won't work. As you might imagine, the better the coatings, the more expensive the binoculars.

With expensive roof prism binoculars, a special feature to look for is anti-phase shifting coatings. Roof prism binoculars with these special coatings on the roof surface will deliver higher-contrast images.

Here are some symbols that are used to describe binocular coatings:

(C), coated optics: one or more surfaces coated.

(FC), fully coated: all air-to-glass surfaces coated. But if any plastic lenses are used, they may not be coated.

(MC), multi-coated: one or more surfaces are multi-layer coated.

(FMC), fully multi-coated: all air-to-glass surfaces are multi-layer coated.

Power and light Every pair of binoculars is engraved with a formula, such as "7 x 35" or "10 x 42." The first number in the formula is the power, or how many times the image is enlarged. With hand held binoculars, as with most things in life, there is a practical limit to power beyond which it is not useful. Depending on the individual, as the power increases, hand tremor begins to degrade the image. Binoculars over 10 power usually require tripod mounting.

The second number in the formula is the diameter of the objective lens in millimeters. The bigger the objective, the more light can enter, and the greater the potential resolution of the image.

Low-light performance is largely dependent on the exit pupil. Exit pupils are the small, bright circles you see in the eyepieces when you hold binoculars away from your eyes and up to the light. They are the actual beams of light coming out through the eyepieces. The exit pupil is calculated by dividing the diameter of the objective lens by the power. A 7 x 35 binocular has an exit pupil of 5 millimeters (35 7 = 5).

At noon, the pupils of your eyes contract to 2 to 4 mm, and at night they may open to 7 mm. If the beam of light exiting the binoculars is wider than the pupil of the eye, the excess doesn't get in: the eye can't see it. During daylight hours things look just as bright through binoculars with 4 mm exit pupils as through those with 7 mm exit pupils. In fact, if they have better coatings, binoculars with 4 mm exit pupils will be brighter.

It's in low light that the larger exit pupil is an advantage. For astronomy, an exit pupil of 7 mm is standard. For birding purposes a 6 mm exit pupil is usually large enough for even the most demanding low-light condition. For daylight viewing, even smaller exit pupils may be more than enough.


As we age, the eye loses its ability to dark adapt. While a 20-year-old person's pupils might open to 7 mm, at 50 years the pupils may open only to 5 mm. Therefore, binoculars with large exit pupils may not help the older birder.

The best birding binoculars are bright as a result of their advanced multi-coatings and top quality optics, which provide brightness you can see all the time, even in daylight.

Will binoculars with a larger exit pupil improve your ability to identify birds at dusk? The best way to tell is to try them, using your eyes.

Field of view The field of view is how wide an area is encompassed in the binoculars' image. It can be expressed as an angle (8), or as the width in feet of the image at 1000 yards (420 ft.) To convert the angular field to the linear field, multiply the angular field by 52.5. Field of view is a matter of eyepiece design. More power usually means a narrower field of view. Wide-field-of-view eyepieces usually have shorter eye relief and may not work for eyeglass wearers. Though it is important, field of view is usually not the first concern in choosing birding binoculars.

Eye relief and eyeglasses Eye relief is how far back from the eyepiece your eye can be and still see the whole field of view. It is the most important consideration for eyeglass wearers, because glasses hold the eyes back from the eyepieces. If the distance to your eyes is greater than the binoculars' eye relief, you will see only the center part of the image. It's like paying for a box seat but watching the game through a hole in the fence.

Normal eye relief for binoculars ranges from 9 to 13 mm. Even though the eyecups of most binoculars fold down to let glasses wearers get closer, in many cases, it's not close enough. If you wear glasses, you need binoculars whose eyepieces are specially designed with long eye relief, which manufacturers describe as 14 mm or longer.

It's odd that eye relief specifications are often left out of brochures and catalogs, since the eye relief number is the best way to compare models for use with glasses. To measure eye relief yourself, point the binoculars' objectives at a bright light source and move a paper back and forth near the eyepieces. The eye relief is the distance from the eyepiece at which the circle of light comes into sharpest focus. This is the optimal place for the eye to be. It's called the eyepoint.

If your binoculars' eyepoint is not compatible with your eyeglasses, you are missing the big picture, a lot of the fun, and possibly the bird.

Birding eyeglasses You may find it helpful to buy a special pair of birding eyeglasses that let your eyes get closer to the eyepiece. For example, my "spare" eyeglasses sport old-fashioned wire-rim frames. The lenses are much smaller and lighter than my regular frames. They bring my eyes closer to the eyepieces than my regular glasses and give me more choice of which binoculars I can use. I can even use 10 x 25 pocket binoculars witIhout feeling as if I'm trying to look through a pair of soda straws.

f your regular prescription includes bifocals, ask your optician to move the dividing line as low as possible, so that you can look over it when using your binoculars. Don't be tempted to eliminate the lower bifocal entirely. You will regret it the first time you try to read your field guide.

The anti-reflective coating technology used on binocular optics is also available for eyeglasses. It increases light transmission when using your glasses with binoculars. It will also improve visual comfort, especially at night, while working under fluorescent lighting, or when using a computer. Once you try it, you'll want it on your regular frames too.

How to choose binoculars Buy the best. Superior optics really pay off for birders in the quality of experience they provide. They will stand up to heavy use and keep their resale value. It's a false economy to buy less.

Try before you buy. Binoculars that are perfect for somebody else may not be the right binoculars for you. See if you can borrow a model you are interested in from a friend and bird with it for a day. Or ask the dealer if you can take two or more models into the field for comparison. If you haven't pre-tested a particular binocular, don't buy it without the assurance that you can trade it in for a different one.

The human eye has a great ability to compensate temporarily for slight misalignment or focus problems. For the few minutes spent evaluating models in a store, a pair of binoculars may look fine. But after an hour in the field you may begin to experience eye fatigue or even a headache. Subtle differences between binoculars may take some time to show up.

The best binoculars will disappear from your awareness while you're using them, so that your attention is on the bird, not the binoculars. The mark of good binoculars is that they make you feel as if you are simply seeing through your own eyes, only closer. You can look through good optics all day long with no sense of strain. With inferior optics, you feel a subtle sense of relief when you stop looking through them.

It pays to invest in the instrument you really want. You will never be sorry you bought the best binoculars.

How to compare models Indoors, where you can control lighting conditions, tape a dollar bill to a wall. Then compare how well various binoculars resolve the same details, both at the center and at the edges of the visual field. Test the binoculars without support, hand holding them as you would in normal use. Is one pair more comfortable to hold? Do you prefer looking through it? You can also test binoculars on a tripod or a beanbag support. You may obtain different results from what you find in the hand-held test.

Try them in the field. Consider the weight. After an hour are they still comfortable around your neck? You may want to try using a replacement strap made of neoprene rubber. Wide and stretchy, it helps to absorb shock and protect the neck.

What to avoid in birding binoculars Zoom binoculars are usually optically inferior to regular binoculars. Avoid fixed focus binoculars. They won't focus up close. And don't get binoculars that focus each eyepiece separately. Individual focus is appropriate for marine binoculars, which require complete waterproofing, but it's too slow on the draw for birding.

What are the best birding binoculars? It's really a matter of personal choice. Different models are best for different purposes and different birders. The 10 power that's tops for distant shorebirds is outperformed by a 7 power with a wider field of view when searching for warblers in dim light.

My personal pick If I had to pick the best one-size-fits-all birding binoculars, they would be the 7 x 42 Zeiss B/GA T. With 7 power magnification, they are universal binoculars that everyone can hand hold. The slim, tapered shape fits large or small hands. Resolution and color accuracy are unsurpassed. The optics are incredibly bright, and a 6 mm exit pupil gives excellent low-light performance. They focus close - 11 ft. An 8.6 field of view delivers a big picture, and they're wonderful, wonderful with eyeglasses. This rugged instrument is armored and won't fog up in wet weather. At 28 oz. it may be a tad heavy for some, but the performance is worth the weight. Their only flaw is that the diopter adjustment is easily turned off its setting. However, other users might consider the ease of re-setting the diopter adjustment a desirable feature.

Lowest priced birding binoculars Included in the table are two examples of very low priced binoculars, the 7 x 35 Bushnell Birder and the 9 x 25 Nikon Travelite III. They get high ratings for the quality of their optics considering their cost. For example, the Bushnell Birder can be bought from some mail-order houses for under $40, including a case and a beginning birdwatching book. No one need be without a loaner.

Zeiss 20 x 60 S stabilization binoculars Birding machismo occasionally requires some birders to boast that they can hand hold 20 power binoculars or a spotting scope. The new Zeiss 20 x 60 S stabilization binoculars make this dream come true for everybody. Push a button, and the prism system inside is free to sway with the movements of the hand. The image stops shaking. How stable is it? Zeiss claims that you can resolve a 15 mm object at 1000 meters without using a tripod. A dime is 18 mm.

This magic is all done mechanically, without any electronics. No whirring gyros to spoil the silence, and no batteries to run down. Good news for eyeglass wearers: the 20 x 60 S offers 14 mm of eye relief, outstanding for 20 power binoculars. The binoculars are rubber armored and come in a high-tech aluminum case. They weigh 3.5 lbs., but this is feather light for typical 20 power binoculars. Especially considering you can leave the tripod behind. And think of the speed! How many times have you missed a bird while setting up your tripod and scope? I'll bet that, even at $4725 a copy, Zeiss can sell all they can make.