Some Binocular Specifications

Magnification and Aperture: A binocular's name describes its magnification and lens size. For example, a "7x50" or "7 by 50" binocular magnifies 7 times and has light gathering lenses (objectives) that are 50mm (2") in diameter.

A binocular that magnifies seven times (7x or 7 power) makes objects appear seven times closer than they do to your bare eyes. A bird 70 feet away seems to be only 10 feet away through a 7x binocular, for example.

The higher the power, though, the more difficult it is to hold a binocular steady enough to keep the image sharp - so the highest possible power is not always the most useful power. 10x is a practical maximum for hand-held use.

A binocular's lens size helps determine how bright the binocular will be, since large lenses naturally gather more light than small ones. You can compare the brightness of identical magnification binoculars by squaring the diameters of their objective lenses. For an example, since a 7x binocular with 50mm lenses (50² = 2500) gathers more than twice as much light as a 7 x 35mm (35² = 1225), the 7 x 50mm binocular is brighter in the dim light of dawn or dusk.

In addition, resolution (the ability of a binocular to show you small details) is inversely proportional to the size of the objective. In other words, the bigger the lens, the smaller the detail it can show.

While big binoculars have advantages, they usually have a big drawback - weight. In general, the bigger the binocular, the heavier it is. A binocular that's too heavy to carry comfortably all day long is not a good binocular, no matter how sharp its optics.

Exit Pupil: A binocular's exit pupil is the circle of light you see in the eyepiece when you hold up the binocular at arm's length and look at a light source. (Don't look at the Sun, as that will damage your eyes.) In general, the bigger the exit pupil, the brighter the image.

To find a binocular's exit pupil size, divide the diameter of its objective lens (in mm) by its magnification. A 7 x 50 binocular has an exit pupil 7.1mm in diameter (50mm / 7 = 7.1).

For the most efficient use of a binocular's light-gathering, match its exit pupil to your eye's largest dilated pupil size. A binocular is brightest when the pupil of your eye is the same size as the binocular exit pupil, since that's when all of the binocular's light is entering your eye.

The pupil of the average youthful eye can dilate to about 7mm, but only in very low light. Accordingly, binoculars with large exit pupils (over 6mm) are recommended only if you are young and do much of your birding in deeply shadowed woods, or at dawn or dusk, when your eyes dilate enough to take advantage of the large binocular exit pupils.

In daylight, your eyes' pupils contract to less than 4mm in diameter. A binocular with an exit pupil larger than 4mm wastes light in the daytime, for as much as 60% of its exit pupil will fall on your daylight-contracted iris, rather than entering your eye. A 7 x 50 binocular, with its 7mm exit pupil, is no brighter during the day than a 7 x 28 binocular with its smaller 4mm exit pupil, since daylight contracts your eye's pupil to under 4mm. In the bright light of day, 7 x 50, 7 x 42, and 7 x 35 binoculars are all no brighter than a 7 x 28. There's no point in buying big binoculars with more light-gathering than your eyes can use if you do little birding in low light. Be sure you really need a large exit pupil before buying a large binocular.

If you observe from a moving vehicle, however - the deck of a boat or a Land Rover on safari - a 6mm to 7mm binocular exit pupil can be helpful, even though a light waster. It's easier to keep your eye fixed on the image in a bouncing binocular if the bino's exit pupil is large.

For dawn or dusk birding, 4mm to 6mm exit pupil binoculars are the most useful.

Binocular exit pupils of 3mm to 4mm are the most usable in average or overcast daylight, as diffused daylight causes your eyes to contract to this size.

Exit pupils below 3mm are mainly for bright daylight use, or for desert or shore birding where you have considerable amounts of reflected light and glare to deal with. You'll find binoculars in this exit pupil range to be of little use in twilight or in dim and overcast conditions.

If you're uncertain which to choose, though, you're better off with a binocular that has an exit pupil too large rather than one too small. It makes more sense to waste some light with a overly-large exit pupil when it's bright, rather than wish in vain for more light when it's dim.

Your eye's ability to dilate declines with age, falling from 7mm to a maximum of about 5mm in late middle age. If you're in this age group, binoculars with 6mm to 7mm exit pupils will be no brighter than a 5mm exit pupil binocular in low light, as some of the light of those overly large exit pupils will fall on your no-longer-fully-dilatable iris and be wasted.

On average, you lose about 1/2mm of maximum dilated pupil size per decade between the ages of 20 and 50, declining to about a 1/4mm loss per decade thereafter - from 7mm or so in your teens, to perhaps 5.5mm by the time you reach 40, to 5mm at 50, etc. Your pupil size will depend on heredity, your general health, and whether you smoke (smokers lose pupil elasticity faster).

There's no point in spending good money for more light-gathering than your eyes can use, so don't go overboard on exit pupil size if you're past middle age.

Relative Brightness: Most manufacturers use a specification called "relative brightness" as a way to compare binoculars. The higher the relative brightness, the brighter the binocular. To find the relative brightness, simply square the diameter of the exit pupil. For example, all binoculars with 4mm exit pupils have a relative brightness of 16 (4² = 16).

Since relative brightness is a mathematical function only, and doesn't take into account aperture differences between binoculars, it should be taken with a grain of salt if used for choosing binoculars. For example, 8 x 32mm and 20 x 80mm binoculars have identical 4mm exit pupils, but they’re hardly identical low light performers. The 625% larger light gathering area of a 20 x 80mm pours 625% more light into its 4mm exit pupil than the smaller lenses of the 32mm binocular. Relative brightness is only useful when comparing the low light performance of binoculars of similar aperture - and then only when your eye's pupil is as large as the exit pupils of the binoculars.

Some manufacturers arbitrarily boost the relative brightness by 50% to derive a specification they call "relative light efficiency." This supposedly takes into account the higher light transmission of coated optics versus uncoated lenses. Since all modern binoculars have coated optics, however, "relative light efficiency" has little justification other than to make such binoculars appear to the unwary shopper to be 50% brighter than binoculars using the more generally accepted relative brightness specification. Relative brightness and "relative light efficiency" are not the same thing. When comparing binoculars using manufacturers' specs, don't confuse an artificially-boosted "relative light efficiency" with relative brightness. We use only the conservative relative brightness figures in our literature so you can compare binoculars on an equal basis.

Twilight Factor: Exit pupil, relative brightness, and "relative light efficiency" comparisons are interesting and often useful, but they're not the best judges of how well binoculars perform in low light. For example, 8 x 32mm and 20 x 80mm binoculars both have 4mm exit pupils, a relative brightness of 16, and a "relative light efficiency" of 24 - but the 80mm binoculars are much better in low light due to their 625% larger light-gathering capacity and higher "twilight factor."

The twilight factor is a more useful judge of a binocular's low light performance than its exit pupil, etc., as it takes into account both light gathering and magnification. Both factors affect how much detail you can see - and seeing detail is what binoculars are all about.

Simply put, the larger an image, the easier it is for you to see details in that image. By the same token, with a smaller image, the brighter it gets, the easier it is for you to see the same details clearly. So, within reason, if magnification goes up, brightness can go down without affecting resolution, and vice versa.

It's like reading a newspaper in the light of a 3-way lamp. If the lamp is at its low 50 watt setting, you have to hold the paper closer (making the image larger) to read the fine print. If the lamp is turned up to 100 or 150 watts, you can hold the paper further away (making the image smaller) and still read the same fine print. In other words, small bright images can show you as much detail as large dim images.

The twilight factor allows you to compare different combinations of aperture and magnification to determine the one that best balances an increase in power against a decrease in brightness (or vice versa). The larger the twilight factor, the better a binocular is in low light.

Twilight factors of 17 and higher are best for twilight or early morning use.

A binocular's twilight factor is found by multiplying its objective lens diameter by its magnification, then finding the square root of that product. An 8 x 32mm binocular, for example, has a twilight factor of 16, while a 20 x 80 has a twilight factor of 40, explaining the better low light performance of the latter despite the identical exit pupils and relative brightness.

As with relative brightness, the twilight factor is a mathematical relationship. It does not take into account light transmission differences between binoculars due to differences in optical coatings, so small numerical differences in twilight factors may not be visible in real life.

And just because an inexpensive binocular and a premium model have identical twilight factors, you cannot assume that their optical performance will be the same. Light transmission differences, distortion, and optical flaws in the less expensive binocular can severely compromise its sharpness and clarity.

So, while it's a useful figure, don't let the twilight factor be your only guide to choosing a binocular. Other factors can be just as important as the ability to make out details in dim light. For example, at first glance, a 10 x 40 might seem better than a 7 x 42 in low light because of its higher twilight factor (20 versus 17.1). But if most of your observing is done in dim wooded areas, the wider field of view and greater depth of field of a 7 x 42 are better for locating birds than the narrower and shallower field of a 10 x 40 - and finding the bird is half way to identifying it.

But, keeping these cautions in mind, the twilight factor still remains a more reliable guide to low light performance than exit pupil or relative brightness.

Near Focus: How close you can get to an object and still see a sharp image of it in your binocular is called the “close” or “near” focus. For general birding, a close focus of 15’ or less is the minimum recommended – letting you observe backyard bird feeders, nests in low trees, etc. However, many birders prefer a close focus of less than 10’ for close-in observing of warblers. Binoculars with close focus capabilities of as little as 4’ are available for butterfly watchers. Birders doing high power long-distance observing – shore birds, raptor-watching – will find a 20’ to 25’ close focus is usually sufficient. Manufacturers’ close focus specifications assume 20/20 vision and individual binoculars can vary a foot or two in either direction, depending on your eyesight.

Depth of Field: This is simply the distance in front of and behind the point of sharpest focus that an image remains usefully sharp in binoculars. Good depth of field minimizes the constant refocusing needed to keep birds sharp and clear as they flit among branches of nearby trees. The higher the magnification of an optical system, and the closer it is focused, the shallower its depth of field will become. It is for this reason that high power binoculars and spotting scopes are usually not well-suited for close-in birding. There is no industry standard for a depth of field specification, so when we test binoculars (as we do with every product we sell before we agree to carry it), one of our tests is for depth of field. We focus at 20'. If the image is sharp enough to read 1/16th inch tall printing located one foot in front of and one foot behind that 20' point, we consider the binocular to have an acceptable depth of field. If the same letters are equally sharp for 18" or more in front of and behind the 20' point of sharpest focus, we consider it to have good depth of field and so note it in our catalog description.

Field of View: The field of view is simply a measure of how wide a piece of the landscape you see through a binocular. It can be expressed either in degrees or in the linear diameter (in feet) of the circular image visible in the binocular at a distance of 1000 yards.

Each time you divide that 1000 yard distance by ten, you divide the field of view by ten. For example, if a binocular has an 8° field of view (420' across at 1000 yards), its field will be 42' at 100 yards, 4.2' at 10 yards, and only 2.1' at 5 yards (only 25 inches across at 15 feet). Similarly, a binocular with a 5° field has a field less than 16 inches wide at 15 feet, and less than 8 inches across at a distance of 7.5 feet (well within the close focus capability of most modern binoculars).

For close-in birding, the wider the field, the better your chance of getting both bird and bird feeder in the field at the same time. At any distance, the wider the field, the better your chance of first finding, then keeping a bird in sight.

A wide field of view is sometimes achieved at the expense of clarity at the edge of the field, however. Don't go overboard on width of field when choosing a binocular unless you know it's sharp from edge to edge.

Your brain will blend the two binocular images into a single circular field when you're focused on a distant object. When you focus on something close to you, the object may appear to be in the center of two overlapping circles of light, particularly with a porro prism binocular.

Eye Relief: The minimum distance between a binocular's eyepiece and your eye that allows you to see the entire field of view is called the eye relief. Long eye relief is important if you must wear eyeglasses to observe, as your glasses can keep you from getting close enough to the eyepiece to see the full field.

Eye relief explainedIf you are near or farsighted, you can usually observe with your glasses off, as binoculars have enough focuser travel to accommodate these conditions. You can also usually leave your glasses off for daylight observing if you have only mild astigmatism, as the pupil of your eye will contract enough to minimize the astigmatism around the periphery of your cornea. Severe astigmatism or severe nearsightedness, for which glasses must be worn, require a binocular with long eye relief (15mm to 17mm or longer) to allow you to wear glasses and still see all, or nearly all, of the field of view.

A binocular with medium eye relief (10mm to 15mm) can be used with eyeglasses, but you'll lose some of the field of view. To visualize this, imagine that the binocular's eyepiece is a knothole in a fence and you're watching a baseball game through the knothole. If you get your eye close to the knothole (or binocular), you see the entire playing field. But, if you move your eye back from the knothole, as eyeglasses effectively force you to do, you see only the central area of the field. The image is still sharp and you're still able to focus properly. It's simply that you can no longer see the whole field.

Excessively short eye relief can prevent your eyes from blending the two circular binocular images into one, giving the effect of looking down two tubes instead of through a window. Therefore, a binocular with eye relief below 8mm or 10mm is not recommended for those observers who must wear glasses.

Unlike manufacturers' technically correct eye relief figures, which are measured from the eyepiece lens (which is usually recessed into the binocular body to protect it from being scratched), the figures in our specifications are the usable eye relief values, determined by rolling down the binocular eyecup and measuring the eye relief from the binocular eyepiece rim, where eyebrows or glasses would touch (which limits how close you can get your eye to the binocular’s eyepiece). Consequently, our real life eye relief figures are often shorter than a manufacturer's, but more realistic.

If you're a bifocal wearer, or only mildly near or farsighted - a birder who could use binoculars with eyeglasses off - you might still prefer a long eye relief binocular that lets you keep your glasses on while observing. This allows you to switch back and forth between checking your field guide and using your binocular, without having to constantly take your glasses off and put them back on. Murphy's Law says that a bird will always fly away while you're taking off your glasses to put your binocular up to your eyes. Being able to use your binocular with your glasses on, even if you don't have to, can keep you from missing a new bird for your life list.

Long eye relief binoculars are generally more comfortable to use in any case, even if you don't wear glasses - as short eye relief can allow your eyelashes to brush the binocular eyepieces annoyingly. This also deposits eyelash oils on the eyepieces that can damage their coatings if not cleaned off regularly.

Interpupillary Distance: This is the distance between the pupils of your eyes, measured from center to center. It is also the distance between a binocular's exit pupils. If a binocular can't fold down or open up enough for its exit pupils to line up with the pupils of your eyes, a shadow will cut off part of the image you see in one eyepiece or the other.

If you suspect that your eyes are more closely or widely spaced than average, make sure that your interpupillary distance falls within the range given for the binocular in which you're interested. If you don't know your interpupillary distance, look through a binocular that you know works for you and measure the distance between the centers of its exit pupils when it's set for your eye spacing.


. . . our 34th year