Binocular Questions & Answers – Oberwerk
Questions? Call us at 937-640-1040 Mon-Fri 9:00am-5:30pm EDT

Binocular Questions & Answers

Welcome to Oberwerk! We’ve tried to answer some of the most common questions we receive regarding our binoculars on this page. But please feel free to call us at 937-640-1040 or email ([email protected]) with your questions. -Kevin Busarow, President, Oberwerk Corp.

10×50? What do the numbers mean?
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.

OK, but what is “12-36×70” and “25/40×100”?
When there is more than one number before the “x”, this means the binocular has multiple levels of magnification. If the numbers are separated by a “-“, such as “12-36×70”, then the binocular is a “zoom”, and the magnification is continuously variable from the first number to the second number (in this case, 12x to 36x), by moving a zoom lever. Note: We do not recommend zoom binoculars. See “Just Say No to Zoom Binoculars” for more information.
On the other hand, if the numbers are separated by a “/”, then the binocular is not a zoom, but uses multiple fixed-power eyepieces to obtain different levels of magnification. Our 25/40×100 Long-Range Observation binocular for example, has 25x and 40x eyepieces on rotating turrets, so you can easily switch back and forth between 25x and 40x magnification. Binoculars using multiple fixed-power eyepieces do not suffer from the optical limitations of zoom binoculars.

Got it. So what’s the magnification of a BT-100XL-ED?
“BT” means “binocular telescope”. Binocular telescopes have interchangeable eyepieces, and it’s the eyepiece focal length that determines magnification. Therefore binocular telescopes don’t mention magnification in their model designation, but do include the objective diameter (100mm in this case). The recommended magnification range for the BT-100XL-ED is from 25x to 90x.

What’s the difference between BAK4 and BK-7 prisms?
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.

What’s “multi-coating”?
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.

What’s the difference between multi-coated and fully multi-coated optics?
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.

What does “broadband multi-coating” mean?
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.

What is “Exit Pupil Diameter”?
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.

Is my age a factor in choosing an astronomy binocular?
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, our 8x56mm model, with it’s relatively huge exit pupil diameter of 7mm, may be no more effective than a binocular with smaller objectives. If your eyes are only dilating to 5mm, then much of that 7mm shaft of light is never reaching your retina.  On the other hand, a larger exit pupil is more “forgiving” of eye placement and IPD setting.  But you may wish to go with a higher-power model that will more efficiently use of your level of dilation- such the 12x60mm model (5mm exit pupil).

What is “eye relief” (also called “Exit Pupil Distance”)?
Eye relief, also known as exit pupil distance, is the distance your eye should be from the ocular (eyepiece) for optimum performance. Eye relief generally decreases as power increases. Low eye relief (less than 10mm) requires you to get very close to the eyepieces, while higher eye relief (greater than 15mm) allows more distance.

I wear glasses- which binocular do you recommend?
Eyeglass wearers need longer eye relief (see previous paragraph) to allow room for their eyeglasses- generally 15mm is the minimum number to look for, and the more, the better. 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.

25x100dducatWill I need to use a tripod with my new binocular?
Tripods are pretty much a necessity for large and/or higher-magnification binoculars, especially when used for astronomy. See the next question for more on this.  Note that for most smaller binoculars, you’ll need an “L Adapter” to attach the binocular to a tripod head.

What is the most powerful binocular I can hand-hold?
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 about 11x. Anything higher will be too “shaky” to get a clear view. For daylight viewing, higher magnifications can be hand-held, any shaking has less of an effect.  The other factor is weight. Binoculars over 4 pounds are going to be much more pleasurable to use when mounted on a tripod. Therefore, our general recommendation for the “most powerful hand-holdable binocular for astronomy” is the 11×70 LW (3 lbs.). For daylight viewing, some customers can hand-hold the 20×80 LW (4 lbs.).  But even though it’s not very heavy, be aware that this is still a really large binocular- it’s 13 inches long. If that’s a little too much, go for the 11″ long 15×70 LW.

What about zoom binoculars?
Absolutely avoid them!  For further information, please read the following: About Zoom Binoculars

What’s the best method for cleaning my binocular’s lenses?
Our favorite optics cleaning method is a Lens Cleaning Pen. This is what we routinely our in our service department. Highly recommended.

Is the view through a “binocular telescope” upside-down?
No, all of the binoculars we sell, including our “binocular telescopes“, are “image-corrected” through the use of prisms- which means you simply see a magnified version of the same view you see without the binoculars.

What is the best binocular for my ocean-front home?homewithview190
If you have the floor space, in front of a big window looking out to sea, the clear choice is the Oberwerk 25/40x100mm Long-Range Observation binocular. For power and ease-of-use, there is simply nothing better even at double the price, here’s why. If you need more magnification than 40x, our Binocular Telescopes, which use interchangeable telescope eyepieces, are the most powerful binoculars in the world. So if you’ve got a million-dollar view, we recommend you narrow your search to one of these.

How important is “collimation”?
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. coll3-600Most 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). coll2-500Proper 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). 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 $2500 Oberwerk BT-100XL-ED using 7mm eyepieces (80x) to a $69 Oberwerk 6.5×32 LW. We have the knowledge, experience, and equipment to do this better than any other retailer.