Handheld Binocular FAQ

There is apparently a lot of dissenting opinion about this, but only one correct answer. The Oxford Dictionary actually says the word ‘binocular’ doesn’t exist in singular form! It only exists in a plural form, along with bellows, forceps, gallows, glasses, pliers, scissors, shears, and tongs. We strongly disagree! All the other words in that list are objects that have two components, both of which are required to function, such as the two halves of a pliers or scissors. We’d add the word ‘pants,’ or ‘pair of pants’ to that list.

But the prefixes ‘bin’ and ‘bi’ both mean ‘two.’ Bin-ocular means ‘two oculars’ (ocular is another word for eyepiece). It’s no different than the word ‘bicycle’, bi-cycle means ‘two wheels’. We don’t ride a ‘bicycles’ or ‘pair of bicycles’, we ride a bicycle. We don’t view the night sky or go bird-watching with a ‘binoculars’ or ‘pair of binoculars,’ we use a binocular.

If you look at the small screenshot below, you’ll see even spell check gets this wrong! The only correct usage of ‘pair of binoculars’ would be ‘a 10×42 Sport ED for yourself, and an 8×42 Sport ED for your spouse’. Additionally, the two components of a binocular can function individually—that’s called a monocular. Same with a bicycle. If just one wheel, that’s a unicycle. We rest our case.

The first number refers to magnification. A “10×50” for example, magnifies the view by 10 times. Objects appear 10 times larger than they do without the binocular.

The second number refers to the objective size (diameter in millimeters). The objectives are the large lenses at the end of the binocular (opposite from the eyepieces). There are roughly 25mm in an inch, so a 10×50 binocular’s objectives are approximately 2″ in diameter. The size of the objective lenses determines the light-gathering power of the binoculars.

Generally speaking, for astronomy binoculars, the larger the objectives, the more you’ll see. For daylight usage, the larger the objectives, the brighter and clearer the view will be. But as objective size increases, the physical size and weight of the binocular increases, and price also goes up almost exponentially.

Chromatic Aberration, often abbreviated as CA, is a type of distortion caused when light passes through a glass lens. The various colors that make up the visible range of wavelengths of light (380-780nm) pass through optical glass at different speeds, which results in various colors coming to focus at different points. This causes a false-color fringing, which is exacerbated by the brightness of the object being viewed, magnification (CA increases with magnification), and focal length of the binocular (CA decreases as focal length increases). Exotic glass types (ED and SD) have been recently developed to reduce the speed difference as light passes through lenses.

BAK4 prisms (barium crown glass) are the highest quality available. BK-7 prisms (borosilicate glass) are also good quality, but brightness falls off slightly at the edge of the field compared to BAK4. Note: All Oberwerk binoculars have BAK4 prisms.

Coatings prevent reflection and scattering of light- which minimizes light loss and offers better image contrast. An uncoated glass surface can lose up to 5% of light transmission due to reflection and scattering. A single layer of anti-reflection coating can reduce loss to about 1.5%. Multiple layers of different anti-reflection coatings can further reduce loss to as low as 0.25%. Multi-coating therefore, provides a higher level of light transmission and image contrast

Many binoculars have multi-coated objectives and oculars, but it’s also important that all internal air-to-glass surfaces are coated (fully coated) or multi-coated (fully multi-coated) as well. A binocular has between 10 to 20 air-to-glass surfaces on each side- so a loss of 5% per uncoated surface could result in a binocular that transmits less than half of the light that enters its objectives! Only the highest-quality binoculars are multi-coated on all glass-to-air surfaces (fully multi-coated). This costs considerably more to manufacture, but allows the highest level of light transmission (up to 95%).

Note: All Oberwerk binoculars are fully multi-coated.

Broadband is the highest-quality multi-coating available.  If you measure the reflectivity of standard multi-coating across the entire range of visible wavelengths (380-780nm), you’ll see that reflectivity increases (more light is lost) at each end of the visible spectrum. Broadband multi-coating has less of an increase in reflectivity at the ends of the range, or in other words a “broader band” of efficiency, across the entire range of the spectrum.

Note: All Oberwerk binoculars are fully broadband multi-coated.

This is the diameter of the shaft of light coming from the binocular eyepieces to your eye. It’s easy to calculate this based on the objective size and the magnification. Exit pupil diameter equals objective size divided by magnification. So for a 20x80mm binocular, the exit pupil diameter would be about 4mm. A larger exit pupil diameter is generally more desirable- especially so when binoculars are used for astronomy, since our eyes dilate in darkness. The wider the shaft of light, the brighter the image will be because the light is hitting more of our retina. A binocular that has too much magnification for it’s objective size will have a darker view, because a narrower shaft of light is reaching a smaller amount of retina.

As we age, our eyes do not dilate as much as they did when we were younger. Younger people (under 30 let’s say) can usually achieve at least 7mm of dilation when dark-adapted. Once we are in our 50’s or 60’s, the eye will typically dilate to only about 5mm or so. Of course there are exceptions to this, many older people can achieve more than 5mm.  But it’s something to keep in mind when choosing an astronomy binocular. If you are in your 60’s, a commonly-found 7x50mm binocular, with a relatively-huge exit pupil diameter of 7mm, may be no more effective than a binocular with 35mm objectives. If your eyes are only dilating to 5mm, then much of that 7mm shaft of light is never reaching your retina.  Oberwerk binoculars generally operate at higher magnifications relative to their objective size, therefore the largest exit pupil we have is 5.25mm.  So for the vast majority of our customers,  choose whatever model you want, and don’t worry about the exit pupil diameter.

Eye relief, also known as exit pupil distance, is the distance your eye should be from the ocular (eyepiece) for optimum performance. If you’re in too close, portions of the field of view will “black-out”, if you’re not close enough, you won’t be able to see the entire field of view. The ocular’s eyecups can to adjusted to help hold your eyes in the optimum position, or can be retracted to make room for glasses.

Binocular manufacturer’s specification for eye relief is almost always overstated, sometimes greatly, due to lenses that are recessed and/or eyecups that take away from usable eye relief. Oberwerk binocular eye relief specifications are typically less than other brands, but that doesn’t mean our eyepieces have less eye relief- we’re simply stating an accurate measure of actual usable eye relief. More into on that here.

Eyeglass wearers need longer eye relief to allow room for their eyeglasses. For Oberwerk binoculars, 12mm of usable eye relief is the recommended minimum, and the more, the better. For other brands (that overstate usable eye relief), the minimum number is around 16mm. If eye relief is not long enough, you won’t be able to see the entire field of view with glasses on. However- many eyeglass-wearers are surprised to find that they don’t need their glasses when viewing with binoculars. As long as your glasses are not correcting for significant astigmatism, you might very well be able to use a binocular without eyeglasses. The focus range of the binocular, with right diopter compensating for any focus difference between the eyes, can often provide the same correction as your glasses.

This will vary with each individual, and involves magnification and size (weight). But the first question is- daylight viewing or astronomy? For astronomy, the most magnification that can be reasonably hand-held is 12x. Anything higher will be too “shaky” to get a clear view, simply because the binocular is also magnifying any shake or tremor in your hands as you hold it. For daylight viewing, up to 15x magnification might be hand-holdable, as any shaking has less of an effect with a daylight scene. But when you’re seeing just a few points of light in the night sky, you need a steadier view to see it clearly.  The other factor is weight. Binoculars over 4 pounds are going to be much easier to use when mounted on a tripod. Therefore, our general recommendation for the “most powerful hand-holdable binocular for astronomy” is a 12×50 (3 lbs.). For daylight viewing, some customers can hand-hold a 15×70 (4 lbs.).

Absolutely avoid them!  For further information, please read the following: About Zoom Binoculars

Binoculars are really two refractor telescopes connected together, with a method to adjust the eyepieces from each telescope to match your own “IPD” (inter-pupil distance). Collimation refers to the alignment of both telescopes to the “hinge”, and therefore to each other. The single biggest problem with the majority of binoculars sold today is that they are not properly collimated. Most people don’t notice minor alignment problems, especially at the lower magnifications of typical binoculars. The brain does a remarkable job of merging images that are misaligned, however during extended viewing sessions, this can cause eyestrain and discomfort (even nausea). Proper collimation becomes increasingly important with larger binoculars that use higher magnification. Oberwerk is the world’s leading manufacturer of binoculars that support extreme magnification (40x and higher)- outselling all other brands combined. At these magnifications, collimation must be as perfect as mechanically possible, therefore we’re the only binocular retailer that has an in-house collimator (actually two of them).  We test and tune, at that same level of perfection, each and every binocular we sell- from a $6300 Oberwerk BT-127XL-SD using 7mm eyepieces (93x) to a $110 Oberwerk 6.5×32 ES. We have the knowledge, experience, and equipment to do this better than any other binocular supplier.