Magnification x Aperture

What does 7x35, 10x50 and 8-16x40, etc. mean, and how do they determine what binocular you want or need?

Magnification:

The 1st number is the magnification factor, the higher the number, the more powerful the magnification. Don't confuse "power" with "good". There are applications where you want and need a higher power, such as 9 or 10, but many others where a lesser power is preferrable.
The magnification factor is always followed by a "x", indicating by how many times larger, or closer, an image will appear over what you'd see through the naked eye (ie: 7x 8x, etc). An image will appear 10 times larger through a 10 x 50 binocular than when viewed without a binocular. Models with a hyphenated number before the "x" are "Zoom" binoculars with variable magnifications.

Why is more magnification not necessarily better? As magnification increases, image brightness and clarity often diminish, the depth of field becomes shallower and the field of view, how wide an area you can take in, usually becomes much narrower. Another concern with higher power units is that they are often bigger and heavier, and any appreciable hand shaking will make their usage uncomfortable at best, useless at worst. Other impediments to proper viewing at high powers are heat wave distortion and other ill effects caused by adverse atmospheric conditions.
Binoculars comfortably used while hand-held can range in power all the way from 3x (often used for theatre viewing; ie: opera glasses) to 16x (A few hand-held compacts actually go up that high). Many larger binoculars are fitted with a hole for screwing in optional tripod adapters, the biggest coming with adapters built in. When choosing such a unit, it is a wise course of action to take the manufacturer's hint and purchase the optional tripod, even though you might not need to use it for every situation encountered. Most tripod adaptable binoculars short of "giant" sized astronomic or marine models will sit safely and securely on any of the myriad ultra lightweight and hiking friendly tripods we carry.

The Aperture (2nd number, following the "x"):

The second number following the "x" refers to the objective lens diameter, measured in millimeters, the same way camera lenses are. This number usually has direct correlation to the unit's physical size, as well as its intended application(s). For instance, an objective lens of 70 or 80mm usually indicates astronomic usage, and almost always require tripod mounting for effective viewing. They are not what you would choose to stick in a camera gadget bag, or carry into the woods for nature study.

On the other end of the spectrum, units with objective lenses in the 20-25mm range are usually pocket sized compacts, perfect for those who need something small enough to tuck into a purse, backpacks, or any camera luggage with room for accessories. Today, compacts come in virtually all the magnification ranges that full size units are made in. while compact models will usually range from 20mm to 25mm in diameter.

Larger objective lenses usually mean more light gathering ability, which generally means greater detail and clearer images. Just a minute difference in aperture can have a great impact on the light your binocular gathers, especially in low/poor light conditions. You will usually find objectives or 50mm or better when shopping for marine or hunting binoculars, because the greater light gathering, or brightnesss factor, is crucial in dark, dusky or heavily shaded areas. The need for greater light gathering for astronomical models over 50mm is fairly obvious.

Those who don't require all of that brightness, such as birders, nature watchers, sporting event attendees, daytime hikers and casual travelers, can take advantage of the smaller, lighter, more easily storable and transportable compacts well below 50mm. Big is not always better when you're looking for a binocular. Everything, the big and the small, have their place, depending on what you want to use your binocular for.

Light Gathering (Exit Pupil/Twilight/Brightness factors):

Though your eyes can adapt to changing light conditions, binoculars cannot, at least not yet. As their light gathering ability is fixed, it is important to choose a binocular suited to whatever viewing conditions you are most likely to encounter.

Most of today's binoculars employ high quality glass prisms, (for example: BaK-4) providing clear, circular exit pupils. Our product descriptions usually denote such features, depending on information supplied by the manufacturers. Glass quality directly affects image clarity across the viewing area. The best glass will deliver a clear flat field across the whole field of view. Expect quality to generally correlate with the price. This fact of life is unavoidable, but when it comes to using an instrument that could strain your two most valuable organs if either inadequate to the task, or if used improperly, saving a few dollars to forego a better model is not a wise decision.

What on earth is an Exit Pupil?

The exit pupil is one indicator of how well an image can be seen, whether conditions are bright/sunny, dusk/twilight or night, etc. The exit pupil is represented in millimeters by the objective lens divided by the magnification factor (ie: for a 7x50 binocular, the exit pupil is 50/7, or 7.14mm). Most of our binocular descriptions provide this number in the specs.

The exit pupil appears as a circular beam of light inside the unit's eyepiece, as you hold a binocular at arm's length, and is the magnified image as it leaves the binocular for your eye. This indicator's relationship to actual eye pupil size is important. The larger the exit pupil, the easier it is to keep the smaller pupil of your eye centered in the exit pupil. This is one reason almost all marine binoculars, for example, have what amounts to the maximum available exit pupils at a 50mm objective (They are usually 7x50, providing a much larger exit pupil than a 10x50 would, 7.1 as opposed to 5). Thus, they permit far better viewing under conditions where it is difficult to hold a binocular steady, such as while out on the water.

In bright conditions, eye pupils usually range from 2 to 3mm. Under such conditions, even the smallest exit pupil binoculars, such as an 7x20 (exit pupil of 2.86mm) compact, will take in more light than your eyes actually need, since the eyes are limiting the brightness. Thus, such compacts provide an image nearly as bright as that produced by larger exit pupil units, such as a 7x50. Under less favorable light conditions, however, such as twilight/dusk or under cover of heavy foliage, eye pupils generally range from 4 to 5mm. Now you can see how the smaller exit pupils of a 7 or 8x20, 10x25, and even 10x50 or 12x50 might be inadequate, depending on how dark your environment is. If you know you will not need the extra brightness, because you only use your binocular in bright daylight, it may be best to take advantage of a compact's smaller size and lower weight. Some applications may require middle-of-the-road abilities, such as birding. Some, not all, birdwatchers need clarity in less than ideal lighting conditions. While a lower exit pupil of 3 or so might serve their needs most of the time, they will find themselves needing the extra oomph of a 4 or 5mm factor. Thus, you will often find that 10x42 and 8x40 are popular ranges for birding. If tracking birds in rain and generally cloudy, dim conditions is your thing, go for the bigger size and better brightness of one of these. Those who need lightness and compact size more than low-light gathering ability are just as well off taking a compact in the 7x20 to 10x25 range.

For general purpose viewing, a minimum exit pupil of 4mm is desirable. Popular ranges for such usage run from 8x32 to 10x42; just the right ability without a lot of needless size or weight.
In addition to twilight or dusk, some need the ability to view in darkness, where eye pupils generally range from 6 to 8mm. For such conditions, this is where the largest exit pupils come into play, such as the 7mm or better exit pupil provided by 7x50, 8x56 or 9x63 models. Though binoculars in these ranges can generally be held by hand, most will be equipped with tripod adapters, making the purchase of a tripod advisable.

Twilight Factor:

The difference in the importance of magnification versus aperture is literally like day and night.
During daylight, magnification is the main factor in image resolution, while at night, when your eye pupil dilates, control shifts to the aperture size. Midway between the two, in twilight, both factors combine to control resolution, via the exit pupil factor just described.

The Twilight factor, easily calculated based on magnification and aperture, indicates performance under these varied conditions. Those with higher twilight factors resolve images better under low/dim lighting conditions. The twilight factor is calculated by first multiplying the magnification by the aperture, and then finding the square root of that product. For example with a 7x50 model, the twilight rating is 18.7 (7 x 50 = 350; Square root of 350 is 18.7). Be aware, however, that Twilight Factor ignores light transmittal and optical quality.

Relative Brightness & Relative Light Efficiency:

These are two more means of analyzing the low light benefits of binoculars. Not all manufacturers provide these figures in their printed specs, but the Relative Brightness factor, like exit pupil and Twilight, is a simple computation, and again, as with those, the higher the number, the brighter the image. Relative Brightness is simply the Exit Pupil squared, ie: an exit pupil of 5 equals Relative Brightness of 25. Relative Light Efficiency is generally Relative Brightness times 1.5, ie: a 25 Brightness factor equals 37.5 Relative Light Efficiency. In addition, certain prism elements or multi coatings can increase this by another 10 to 15%. Among the binoculars providing this additional Relative Light Efficiency are those employing Barium Crown glass, identified in our product descriptions as BaK4. Now, whether you see one factor being used in a model's specifications, or another, you can still compare apples with apples, and marine binoculars with marine binoculars, making your decision on what to buy as simple as your intended use, budget and comfort level.

Field Of View (FOV):

Field of view measures the widest area visible, and will vary from one model to another, unlike the afore mentioned factors, the main determinant being eyepiece design. One general rule does apply, however; field of view decreases as magnification increases, and a 10 power unit will almost always have a narrower FOV than an 8x, as an 8x in turn would have in comparison to a 7x, etc. It works the same way with a camera lens; the wider the lens, the more area you take in, but conversely, the smaller everything is. Telephoto lenses, like higher power binoculars, make your central subject larger, but leave out more of its surroundings.

Wide fields of view are important for close observation in deep wooded areas, tracking migrating flocks or fast moving action where you want to have adequate time to reposition yourself before your subjects move "off screen". Like any other factor, or feature, wide is not always better. Smaller subjects can easily get lost in the crowd of a wide view and many applications demand a more narrow focus for effectiveness. Another limitation of most wide angle models (Usually identified by a WA somewhere in the model name, and grouped together for easy location) is that they lack long eye relief, limiting their use with eyeglasses. They also tend to be bigger and heavier than normal FOV equivalents, with lowered sharpness at the edges. Also be aware that a "Wide Angle" 10x50 will simply provide more coverage than the average 10x50. It will not necessarily cover more area than a "Non-wide" 7x50.

Field of view can be measured either in feet at a thousand yards (Or meters at a thousand meters), or in degrees (How much of your full 360° surroundings are you viewing). Most manufacturers provide the first measurement to us in feet and virtually all our specifications include this factor. The alternate method by degrees is a simple calculation based on the feet given. Every 52.5' equals 1° @ 1,000 yards, therefore a binocular with a foot measured FOV of 300 has a degree measured FOV of 5.7°.

Eye Relief, "Long" Eye Relief and using binoculars with eyeglasses:

The full Field of View (FOV) is visible when your eyes are a given distance away from the eyepieces. This distance is called Eye Relief, and varies in range from 5mm on up over 20mm. For comfortable and practical viewing of the full FOV while wearing eyeglasses, you need at least 15mm of eye relief. Many newer models feature versatile folding rubber eyecups, allowing the binoculars to be brought closer to your eyes. You can always extend foldable eyecups for viewing without glasses.

When to use glasses and when not to: Those with moderate to severe astigmatism generally need to keep their glasses on, thus making it imperative to choose a model with adequate eye relief. We make that a snap by grouping all Long Eye Relief models together. For those with Far or Near-sighted vision, viewing through their glasses can be easier, but is not normally crucial, since the focusing on most binoculars compensate for their conditions. If you tend to pull your binoculars on and off with any frequency, it can become a hassle to continuously put your glasses on and off, especially out in the field. With so many Long Eye Relief models available, it usually makes sense to just get one of them and have the versatility to use it either way.

Porro Prism versus Roof Prism designs:

Porro Prism models are what most people picture when they think of a classic traditional binocular, with two offset barrels connected by a center axis rod. Porro prisms tend to provide wide fields of view and crisp, sharp images. A trade-off with the larger size, more traditional Porro Prism models is that they tend to be bigger and heavier than their Roof Prism counterparts, although newer designs and Reverse Porro Prism technology are eliminating this disparity. The classic Porro Prism design has enjoyed a special niche in military and law enforcement applications.

Roof prisms are usually more compact and stylish looking, without gaps between the barrels and center axis. Their tighter design also tends to provide more durability, and renders them less susceptible to contamination by the elements.

Which one to get? Since many ranges of power and aperture are available in both styles, the choice might come down to how comfortable the traditional Porro design feels as opposed to the sleeker Roof design. Another factor might be price, as the typical Roof Prism model will usually be more expensive than its closest Porro counterpart. Even style and looks are legitimate factors, if you desire the more modern, slimmer and cleaner lines of a Roof Prism, or the classical look and feel of a traditional Porro.

Other factors to consider:

Rubber armour coating: Many binoculars are encased in a rubberized coating. Some, not all of them, are either waterproof, or weather resistant. Rubber armouring can serve any or all of several purposes, from increasing shock and impact protection to helping keep moisture away from sensitive areas. The only universal benefit is better gripping and easier handling, and this benefit can be of extreme importance to those who want a more comfortable, sure grip, especially in conditions where cold or sweat can make gripping a hard plastic or metal surface a misery. Never assume, however, that rubber armouring means impact or moisture protection. Waterproofing (and fogproofing) are accomplished via special seals, and the presense of rubber armour on such models is purely incidental.