Porro Prism

Roof Prism

Reverse Porro Prism

Magnification Power
On the upper surface of your binoculars you'll find two numbers, for example 7X35 or 10X40. The first number is the magnification power of the binoculars; 7X (pronounced "seven ex" or "seven power") means the binoculars make subjects appear seven times closer than without magnification. The second number is the diameter of the binoculars' objective lenses in millimeters, and is related to the binoculars' brightness.

Some birders prefer binoculars as powerful as 10X, especially for viewing hawks, waterfowl, and shorebirds, which are likely to seen at a distance in relatively open areas.

For general birding though, most bird watchers prefer 7X or 8X binoculars, for a couple of reasons. First, the more powerful your binoculars, the harder they are to hold steady for comfortable viewing. "Hand shake" gets worse in binoculars over 8X. Second, lower power binoculars tend to have better light-gathering ability and a wider angle of view than more powerful models.

Avoid "zoom" binoculars, whose magnification varies, for instance from 7X to 15X. They're often bulky and heavy, and their optics are often poor. At the higher magnifications they produce a dark image and are hard to hold steady.

"Image Stabilizer" binoculars have moveable internal prisms that counter the vibrations caused by unsteady hands. The technology is promising, but to date these binoculars still need better optical quality and light-gathering ability.

Light-gathering Capacity
To a birder, binoculars' light-gathering capacity is nearly as important as image sharpness. Only a bright image reveals subtle field marks and the full beauty of bird colors.

Light enters binoculars through the objective lenses, those farthest from the eye. The diameter of these lenses in millimeters is the second number in the binoculars' designation, so 7X35 binoculars, for instance, have 35mm objective lenses. The bigger the objective lenses, the more light gathered and the brighter the image. 7X50 binoculars have the same magnification power as a pair of 7X35s, but the 7X50s, with their 50mm objective lenses, have greater light-gathering ability. Just as an owl's large eyes gather sufficient light to allow nocturnal vision, binoculars with large objective lenses permit birding in low light, such as dawn or dusk, or in dark forests.

The best measure of a binocular's brightness is the size of the exit pupil, which you can see by holding your binoculars at arm's length and looking into the eyepieces. A small exit pupil appears as a dark hole, whereas a large one is a brilliant, clear circle.

To calculate the exit pupil size, divide the objective lens diameter by the magnification power. For example, 7X35 binoculars have an exit pupil of 5mm, whereas 7x50s have an exit pupil of 7.1mm, which provides a much brighter image. Avoid binoculars with an exit pupil of less than 5mm, which give a dim image, useful only under bright lighting conditions.

Optical Quality

An optical quality test
Even binoculars with large objective lenses can have poor light-gathering ability if the optics are poor. Examine the edges of the exit pupil to see if it forms a complete, bright circle or if it is partially shaded in gray. If only the center of the exit pupil is bright, then inferior optics are blocking some of the light, counteracting the advantages of the large objective lenses.

Coated optics
Light entering the objective lens passes through up to eight pieces of optical glass in each barrel. At each glass surface some light is reflected backward rather than passing through the prisms and lenses. The optics of well-made binoculars are coated with non-reflective film that helps deliver more than 90% of the light gathered by the objective lenses. Without non-reflective coating, binoculars may lose as much as 60% of that light.

Although you should never look at the sun directly through binoculars, coated optics also help when you're looking at backlit subjects. Within uncoated binoculars, reflected light causes annoying glare.

Always choose binoculars with "fully coated" optics. Beware that, although most manufacturers coat exterior lenses, the interior optics of some inexpensive binoculars may be uncoated, causing a significant loss of light. Also note that the quality of coating varies; price makes a difference, with more expensive binoculars having better quality optical coating.

Resolution
The quality of the optical glass used in binocular manufacture affects resolution (the ability to separate objects visually) and image sharpness. High quality optical glass is extremely expensive, and each lens and prism must be professionally ground and mounted with expert precision. Top-of-the-line binoculars usually have excellent optics, producing images that are crisp across the entire field of view. Manufacturers of lesser products cut corners, often by using cheaper glass and looser quality control.

To check the center-to-edge resolution of a pair of binoculars, focus them on a map or newspaper tacked to a wall. Stand back about 25 feet and see if you can read the print at both the center and edge of the field of view.

Field of View and Focus Distance

Field of view
Field of view is the width of the area you see while looking through your binoculars. It is usually expressed as the width in feet of the area visible at 1,000 yards from the observer, although some binocular manufacturers give field of view in degrees.

The wider the field of view, the easier it is to locate birds with your binoculars. Wide-angle binoculars are especially useful for beginning bird watchers, because the larger field of view makes it easier to find birds. Extra-wide-angle binoculars provide an even broader field of view by using more and larger lenses in their optical system, but this makes them expensive and heavy. Beware of inexpensive extra-wide-angle binoculars; they are probably sharp only in the center of the field. Many experienced bird watchers find that a standard field of view is adequate for most situations and that extra-wide-angle binoculars are unnecessary.

Close-focusing ability
If your bird watching tends to be at close quarters, or if you like to study butterflies as well as birds, close-focusing binoculars may be a good choice for you. A number of modern models, usually roof-prism designs, can focus as close as four feet.

Alignment and Durability
Binoculars actually consist of two separate optical instruments, an individual telescope for each eye. So it is essential that they stay in alignment. When binoculars are functioning properly, both sides focus on the same field of view, but a sharp jolt can easily throw them out of alignment so that the two fields no longer overlap. If the binoculars are severely misaligned, you will see a double image when both your eyes are open, and the subject will look blurred. Binoculars that are only slightly out of alignment may actually be a worse problem, because your eyes strain to bring the two images together, leading to eye fatigue and a headache.

Checking alignment
To check the alignment of your binoculars, try this simple test:

In alignment
Look at the roof of a house with them, then, continuing to look through the eyepieces, move the binoculars about eight inches away from your eyes. If the binoculars are in alignment, the horizontal line of the roof should be at the same level in both fields.

Out of alignment
If the roofline appears offset the binoculars are out of alignment.

Realigning binoculars is no simple task; it should only be done by an experienced technician.

Durability
You get what you pay for with binoculars, as with any other consumer item. Inexpensive binoculars are more likely to go out of alignment than higher-priced models, because their prisms and lenses may be glued in place rather than securely held by metal brackets. Temperature changes or slight jars can easily throw inexpensive, poorly-made binoculars out of alignment. Birding can be very tough on optical equipment; it makes more sense to invest in rugged, well-made binoculars initially than to repeatedly replace or repair less durable ones.

Eyeglass Wearers and Eye Relief

Keep your eyeglasses on
If you wear eyeglasses, leave them in place when using binoculars; it's hard to find and focus quickly on a bird if you always have to remove your eyeglasses first. Plus, if you have astigmatism, you will lose the correction your glasses provide if you remove them, sacrificing image sharpness.

Eye Relief
Older binoculars required the eyes to be close to the eyepiece to see the full field of view. Wearing glasses prevented this, giving a view like looking down a tunnel and resulting in eye strain.

Many modern binoculars are more eyeglass-friendly. They overcome this problem by featuring long eye relief. The optics direct the focal point further back behind the eyepiece so the eye-glass wearer can see the complete field of view.

Long eye relief binoculars are usually identified as such by the manufacturer. Look for the amount of eye relief, given in millimeters, in the model's technical specifications. Most eyeglass wearers need 12 to 15mm of eye relief to be comfortable.

Eyecups
Non-eyeglass wearers should use the binoculars with their eyecups in the raised position. Some designs have folding rubber eyecups; other designs snap up and down. The recently-developed turn-and-lock style of eyecup works well. Try binoculars with various eyecup styles and see which works best for you.

Shopping for Binoculars
Once you've narrowed down your choice of binocular style, magnification power, objective lens size, and field of view, try the following tests on the array of suitable binoculars in your local optics store. Save your final decision regarding price until you've examined what's available.

Compare binoculars of the same magnification power by holding one above the other. Alternately look through each binocular, comparing them for brightness and clarity. Then compare the best binoculars from your first selection with a third group, each time choosing the binoculars with the best characteristics. Continue this process of elimination until you have thoroughly examined everything that's available

Check optical quality by holding the binoculars at arm's length, and checking the exit pupils to see if they are blocked at the edges by gray shadows. Carefully examine the print on a billboard or sign to see if you can read the lettering at the edge of the field as well as at the center.

Check that the binoculars have "fully coated" optics; all optical surfaces should be coated with an even purple-violet or amber hue. Carefully examine the objective and eyepiece lenses for scratches.

Be sure that all the mechanical parts move smoothly and that the bridge supporting the barrels does not wobble.

Outside the store, check alignment by looking at a rooftop or horizontal power line.

Look at the edge of a backlit sign or building to see if it is fringed with a band of bright color. This fringing indicates an inferior optical system that cannot focus light of different wavelengths to the same point.

If you wear eyeglasses, be sure the binoculars feature long eye relief.

After narrowing the field to a few choices, select the best binoculars you can afford.

Cleaning and Protecting Binoculars
Your binoculars face many hazards on your birding adventures. Cleaning your binoculars and protecting them from damage will help them last longer and function optimally.

Cleaning Binoculars:
Thoroughly wipe off metal parts.
Lightly brush lenses with a wad of lens-cleaning tissue or a soft camel's-hair brush to dislodge particles of sand and grit. Removing this debris prevents the lenses from becoming scratched during cleaning. Hold binoculars upside down so that dirt will fall away from the lens surface.
Fold a piece of lens-cleaning tissue so that it is at least four layers thick. This prevents oil from your fingers from soaking through the lens tissue and onto the lens surface. Gently wipe lens surfaces with a circular motion.
If there is a film of oil on the lens, put a drop of lens cleaner on the tissue and repeat the circular wiping movement.
Look for dirt on all the internal optics by holding the binoculars up to the light and looking into the objective lenses.
Never try to open the binoculars; you can easily disrupt their alignment.
Although it's expensive, leave internal cleaning to the professionals.

Protecting Binoculars:
Always keep your binoculars around your neck in the field. Never swing binoculars by the strap as you walk. Accidentally banging them on a tree could throw them out of alignment.
Always tuck your binoculars inside your jacket or secure them under your arm when you have to perform an active maneuver like jumping across a ditch or climbing a rocky slope.
Never leave your binoculars on your car seat. A sudden stop may send them flying, a sure way to knock them out of alignment.
Never leave your binoculars uncovered in your car, especially on a hot day. If thieves don't find them, the sun may soften the lens coatings, causing them to crack and separate from the lenses.
Protect your binoculars from rain. Water can leak into the housing, causing internal fogging and carrying in dirt, which can stain the internal optics. Rain guards help during light rain and drizzle, but they are not adequate protection for heavy rain.
If your binoculars do fog up on the inside, set them in a warm, dry place, and they will probably dry out in a couple of days. Or leave them overnight in a sealed bag with some desiccant (available from a camera store) that will absorb the excess moisture. Otherwise, fungus may start growing on the lens coating. Take desiccant on birding trips to humid regions, where your binoculars may not dry out on their own.
If your binoculars fall into fresh water, have them professionally cleaned as soon as possible to avoid rusting. If you drop them in salt water, rinse them thoroughly in fresh water, seal them in a plastic bag, and rush them to a professional service department immediately. Salt water can quickly ruin binoculars.

Calibrate Binoculars For Your Eyes
Most binoculars have a center focusing wheel that adjusts the focus of both eyepieces simultaneously and a separate diopter adjustment that allows you to focus one eyepiece independently, to make up for the differences in vision between your left and right eye.

To determine the correct diopter adjustment on your binoculars, stand about 30 feet away from a sign with clear lettering, making sure that it is in the middle of the focal range of your binoculars.

Follow these five steps:

Step 1:
Notice that the two binocular barrels pivot on a hingepost allowing the eyepieces to fit your eyes comfortably. Facing the sign, spread the barrels apart as wide as you can. Then, put the binoculars to your eyes and press the barrels together until the two images converge into one. If you cannot push the eyepieces close enough together to see through comfortably, reject those binoculars; the "interpupillary distance" of that model may be too wide to accommodate the narrowness of the space between your eyes. The number (angle) indicated on the hinge post will always be the same for your eyes, on any pair of binoculars.

Step 2:
Turn the center focusing wheel counterclockwise, as far as it will go. Locate the diopter adjustment ring. On most binoculars it is a ring on one of the eyepieces (usually the right one) that is marked with calibrations and can move independently. (Note that some binoculars have a separate knob in the center or another mechanism for diopter adjustments; if so, consult the manufacturer's instructions.) Turn the diopter adjustment ring in a counterclockwise direction untiI it stops. Now both eyepieces should be out of focus.

Step 3:
Facing the sign, lift the binoculars into position and cover the end of the right binocular barrel. With both eyes open, turn the center focusing wheel until the lettering comes into sharp focus. To be sure you have the sharpest possible focus, pass the sharpest point and then back up to find it again.

Step 4:
Now cover the left barrel (keep both eyes open) and turn the diopter adjustment ring clockwise to bring the lettering into focus. Be sure to leave the center focus in exactly the same position as before. Pass the point of sharp focus and then back up to where the lettering is sharpest. Uncover the left barrel and the binoculars should be in perfect focus and calibrated for your eyes.

Step 5:
Note the diopter setting because it is now adjusted to your eyes. That setting should remain constant, unless your vision changes. Some people put a piece of tape over the diopter adjustment to prevent it from shifting accidentally. Once this adjustment is set, you need only adjust the center wheel to focus both eyepieces.

Visit Shaw Creek Bird Supply and see our wide selection of Audubon Binoculars.