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Tucson Audubon Society
Binoculars for Birding
Recommendations from the Tucson Audubon Nature Shop
Remember: The Main Nature Shop carries one of the best selections of binoculars in Tucson. Please stop in our shop when you're searching for the perfect pair. As an added bonus, if you're a member you receive 10% off your binocular purchase!
Binoculars
for Birders
Birders demand a lot from a pair of binoculars. Not only do we want to see all the detail
in a bird’s plumage, but we also want the color bright and true.
We want to sit on a bench and observe the tail feathers of a hummingbird and then within seconds focus on a hawk perched in a distant tree. And we do this in a wide variety of conditions and climates — from rainstorms to snowstorms, from bright deserts to cloudy rainforests, and from dawn to dusk and beyond. Unfortunately, there is no perfect binocular and some compromises must always be made. The Audubon Nature Shop tries to carry binoculars that meet most birders’ needs at a price for every pocketbook.
When you first shop for binoculars all the numbers and jargon can be confusing. This article is an attempt to take some of the mystery out of buying binoculars. There’s a lot to learn and it can be overwhelming, but read this article and then stop by and chat with our Nature Shop volunteers and staff for additional help in unraveling the mystery of binoculars.
Types of
Binoculars
There are two basic types of binoculars: Porro prism and roof prism. You can tell them
apart by the shape. Porro-prism Binoculars: These have an angled design. When standing on
their barrels they appear to form an “M “shape. Porros focus using an external
focus wheel which, when turned, moves the eyepieces forward or backward along an external
barrel. Roof-prism Binoculars: This design features two straight barrels, giving it an
“H”shape. With this design the focusing mechanism is enclosed inside the
binoculars and is adjusted with an external focusing knob or wheel.
Roof-prisms cost more to manufacture and tend to be more expensive than porros, but roof prism binoculars are not necessarily better than porro prism binoculars. Quite often when comparing inexpensive, similarly priced binoculars the porro prism is the better product. The advantages of roof prisms are: the narrower body shape is easier to handle; there are fewer external moving parts (this increases durability; they are generally water proof; and they produce an apparently larger image.
Binocular
Anatomy
Binoculars are made up of a lot of parts, but you only need to know a few to
discuss them intelligently. The eyepiece (or occular lens) is the lens closest to your
eye. The objective lens is the lens closest to the object you are viewing. The focusing
wheel is usually located along the center hinge. The diopter setting is usually located on
the right eyepiece, but can be along the center hinge.
The Numbers
There are a lot of numbers and math associated with optics. Some are important
and some are dependent on so many other factors that they aren’t worth worrying
about. The numbers written on the body of the binocular tell you much of what you need to
know when shopping and comparing binoculars. What you’ll see is: 7x35, 8.5° or 7x35,
446’ at 1000 yards.
The first number is the size of the binocular’s magnification. With a 7x35 binocular the image is 7 times larger than what you see with the naked eye. Good birding binoculars range in power from 7x to 10x. Magnification generally has little influence on image quality. The image just appears smaller with a 7x than with an 8x or 10x. The amount of detail you see is determined by many other factors. Often beginning birders look only at magnification at the expense of other features.
The second number (35) refers to the diameter of the objective lens in millimeters. A larger objective lens takes in more light thus enhancing brightness and resolution of the image. The larger the diameter of the objective, the more light will be allowed in and the more detail you can see.
The other numbers (8.5° or 446’ in our example) represent the field of view. This is the width of the area that you can see through binoculars at 1000 yards. It is usually expressed as degrees of an angle or as feet. In our example if you were looking at a fence line 1000 yards away, you could see 446 feet of the fence through your binoculars. One degree is equal to about 52.5 feet at 1000 yards. To convert an angle to feet multiply the angle by 52.5 (8.5x52.5 = 446.25). Generally the size of the field of view decreases as magnification increases, but other factors are also involved. Manufacturers can engineer the eyepiece to give a wider field of view or change the design and shape of the binocular’s lenses to create “wide angle” binoculars. A wide angle binocular can be useful for beginners or children since it’s much easier to find an object when you can view more of its surroundings.
Other
Numbers
Exit pupil is the size (diameter) of the beam of light that is delivered to your
eye by a binocular. The larger the exit pupil the brighter the image. You can calculate it
by dividing the size of the objective by the magnification (35 / 7 = 5mm). A large exit
pupil is an advantage in low light conditions. There is a limit to this however. A normal
eye will dilate to about 5-7 mm in low-light conditions and excess light (greater than
5-7mm) from a larger exit pupil will not be able to enter the eye.
There are many other calculations that can be used to express differences between models such as relative brightness and twilight factor. Relative brightness is the square of the exit pupil. This is an attempt to quantify brightness. The twilight factor is calculated by multiplying the magnification by the diameter of the objective lens and taking the square root of the result. This calculation shows how resolution in low light can be improved with higher magnification.
The simplest and most meaningful measure of brightness is the exit pupil. Beyond this, what you want to look for are better optics (lenses and prisms) and lens coatings which will improve transmission of light.
Other
Considerations
For birding and wildlife viewing close focus can be very important. It is the
distance from which a near object can be observed and still remain in sharp focus. Ideally
a pair of binoculars will focus on objects at 10 feet or even closer. Compact models
usually have a good close focus, but you sacrifice brightness, power, and field of view.
Many binoculars now are manufactured with the birder in mind and can focus down to 6 feet.
An important consideration for eyeglass wearers is eye relief. The maximum distance your eye can be from the eyepiece and still see the complete field of view is termed eye relief and it is usually expressed in millimeters. If you wear eyeglasses, 12-15 mm will allow you to see 70-80% of the field, and 20 mm will allow you to see the whole field. Almost all binoculars come with some eyecups that fold down or screw in to allow eyeglass wearers to get as close as possible to the eyepiece.
The naked eye is able to focus on near or far objects without a conscious effort. When we use binoculars, everything changes, and we have to manually focus on objects as we or they move about. With binoculars the area in which your eye can perceive a satisfying impression (without refocusing) of an object as it moves slightly back and forth is called the zone of acceptable focus or the depth of field. It can be annoying to have to continually fiddle with the focus wheel while viewing birds sitting close together, one behind the other. Depth of field usually decreases with increased magnification, but a wide angled eyepiece can increase the apparent depth of field.
Optical
Coatings
Optical coatings and lens quality are what create the wide variance in binocular
prices. When you do the math on a pair of $60 Bushnell Birders and a $1,500 pair of
Swaroski ELs, the numbers could be close, but you can see the difference when you look
through the binoculars. Sophisticated manufacturing techniques, lens coatings, and fine
quality lenses make a binocular expensive.
As light passes through glass, some is reflected back. That’s why birds occasionally fly into windows. There are 10 to 16 glass surfaces in most binoculars, and each time light passes through one of these the amount of light reaching the objective lens becomes less and less. Also, the scattered light bounces around inside the binoculars creating a hazy image and blurred colors. Optical coatings can reduce this effect. There are several terms for methods of coating. A fully multi-coated lens will let through the most light and is the most expensive. The types are:
Coated: selected lens and prism surfaces are coated to improve light transmission.
Fully Coated: all air to glass surfaces are coated.
Multi-Coated: one or more surfaces of one or more lenses have been coated with multiple films.
Fully Multi-Coated: all air-to-glass surfaces have been multi-coated.
# # # #
All this information should make you more comfortable shopping for binoculars. But remember, technical information is not the only criteria for finding the binocular that’s right for you. It needs to feel good in your hands, fit properly to your face, and have a weight that’s as comfortable for you at 6 p.m. as it was at 6 a.m. You also need to know how to use and care for your new binoculars.
Bird questions? Check Birding | General questions? Contact: Tucson Audubon Society | Webmaster: Email
This page was updated on 06/07/06