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Home Page > Binocular Info and Support > Frequently Asked Questions
10x50? What do the numbers mean?
The first number refers the magnification of the general purpose or astronomy binoculars. A "10x50" for example, magnifies the view by 10 times. Objects appear 10 times larger than they do without the binoculars.
The second number refers to the objective size (diameter in millimeters). The objectives are the large lenses at the end of the binoculars (opposite from the eyepieces). There are roughly 25mm in an inch, so a 10x50 binocular's objectives are approximately 2" in diameter. The size of the objectives determines the light-gathering power of the binoculars. Generally speaking, for general purpose or 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 binoculars increase, and price also goes up almost exponentially.
OK, but what is "12-36x70" and "25/40x100"?
When there is more than one number before the "x", this means the binoculars have multiple levels of magnification. If the numbers are separated by a "-", such as "12-36x70", then the general purpose or astronomical binoculars are "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 binoculars are not zoom, but uses multiple fixed-power eyepieces to obtain different levels of magnification. Our 25/40x100's for example, have 25x and 40x eyepieces on rotating turrets, so you can easily switch back and forth between 25x and 40x magnification. Note that these types of general purpose or astronomy binoculars are typically of the very highest quality, and do not suffer from the optical limitations of zoom binoculars.
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: Every pair of general purpose or astronomical binoculars we offer has 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 astronomy binoculars lens and fully multi-coated optics?
Most quality astronomical 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. Only the highest quality binoculars are multi-coated on all glass-to-air surfaces (fully multi-coated). This costs more, but allows the highest level of light transmission. Note: All Oberwerk, Fujinon and Miyauchi binoculars brand/model we offer are fully multi-coated.
What is "Exit Pupil Diameter"?
This is the diameter of the shaft of light coming from the general purpose or astronomical binoculars to your eye. It's easy to estimate this based on the objective size and the power. Exit pupil diameter roughly equals objective size divided by power. So for our 15x60mm binocular, the exit pupil diameter would be about 4mm. A larger exit pupil diameter is generally more desirable- especially for usage of astronomical binoculars, since our eyes will dilate in darkness. The wider the shaft of light, the brighter the image will be because the light is hitting more of our retina. The reason high-power binoculars have a darker image is because a narrower shaft of light is reaching a smaller amount of retina. Note: Our specifications for Exit Pupil Diameter are from our actual measurements- not the factory published numbers.
Is my age a factor in choosing general purpose or astronomy binoculars?
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 somewhere around 7mm of dilation. 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, but it's something to keep in mind when choosing general purpose or astronomy binoculars. 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 general purpose or astronomy binoculars with smaller objectives. If your eyes are only dilating to 5mm, then much of that 7mm shaft of light is never reaching your retina. In this case, you may wish to go with a higher power model that will more efficiently use of your level of dilation- such the 12x60mm model.
What is "eye relief" (also called "Exit Pupil Distance") rating on a pair of general purpose or astronomical binoculars?
Eye relief, also known as exit pupil distance, is the distance the pupil 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- what general purpose or astronomical binoculars do you recommend?
Eyeglass wearers need a higher amount of eye relief (see previous paragraph) to allow room for their eyeglasses- generally 15mm is the minimum number to look for. 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 pair of Oberwerk, Miyauchi, or Fujinon astronomy binoculars without eyeglasses, due to the extra focus range of the binoculars.
Will I need to use a tripod with my new binoculars?
Tripods are pretty much a necessity for large binoculars, especially when used for astronomy. See "What is the most powerful general purpose or astronomical binoculars I can hand-hold". Note that for most smaller general purpose or astronomical binoculars, you'll need an "L Adapter" to attach the binocular to a tripod head.
Is "ruby" coating available on your general purpose or astronomy binoculars?
No. First of all, the coating has nothing to do with the mineral ruby. Some manufacturers filter red to compensate for poor-quality optics that do not properly converge the color spectrum. By eliminating red from the spectrum, the optics appear to do a better job of minimizing color abberations- but gives the view through the general purpose or astronomy binoculars an unnatural greenish cast. Steer clear of "ruby" coating- it's simply a gimmick with no redeeming qualities. We've seen ads from unscrupulous vendors that claim magical "night vision" properties for their "ruby-coated" general purpose or astronomical binoculars - "no batteries required". Rubbish...
What is the most powerful general purpose or astronomical binoculars 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 12x. Anything higher will be too "shaky" to get a clear view. For daylight viewing, higher magnifications can be hand-held, the shaking has less of an effect. The other factor is weight. General purpose or Astronomical binoculars over 4 pounds are going to be much more pleasurable to use mounted on a tripod. Therefore, our general recommendation for most powerful hand-holdable binocular for astronomy is an 11x70 (3 lbs.). For daylight viewing, it would be the 20x80LW (3.5 lbs.). But even though it is not very heavy, be aware that this is still a really large binocular- 13 inches long in fact. If that's a little too much, go for the 15x70.
What about zoom binoculars?
For further information regarding zoom binoculars, please go to: Zoom Binoculars
What does "broadband" mean?
Broadband is a type of multi-coating, and is the highest-quality multi-coating available from our factories. If you measure the reflectivity of standard multi-coating across the entire range of visible wavelengths (380-780nm), you'll see that reflectivity increases (is less efficient) at each end of the 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 visible spectrum. Note: Every pair of Oberwerk binoculars is fully broadband multi-coated.
Does the color of the lenses tint the view through the binoculars (like sunglasses)?
High-quality multi-coatings typically give the lenses a subtle green and/or purple appearance- but because they are so highly-efficient, transmitting at least 99% of the light, they will not tint the view through the general purpose or astronomy binoculars. The exception are poor-quality (highly-reflective mirror-like) coatings, which can reflect away part of the color spectrum. For example, so-called "ruby" coating reflects away most of the red, which gives the view an unnatural green cast.
What's the best method for cleaning my general purpose or astronomical binoculars lens?
Our favorite optics cleaning method is the LensPen. This is what we routinely our in our service department. Highly recommended. Buy them here.
Is the view through a "binocular telescope" upside-down?
No, all of the discount binoculars we sell, including "binocular telescopes", are "image-corrected" through the use of prisms- which means you simply see a highly-magnified version of the same view you see without the binoculars.
What are the best binoculars for my ocean-front home?
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. There is simply nothing better even at double the price, here's why. We recommend that you visit our sister store at GiantBinoculars.com, which specializes in discount binoculars specifically for the home-owner with a view.
How important is "collimation"?
General purpose and astronomy binoculars consist of two 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 general purpose and astronomical 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 general purpose and astronomy binoculars. The brain does a remarkable job of merging images that are misaligned, however during extended viewing sessions, this can cause eyestrain and some discomfort (even nausea). Good collimation becomes increasingly important with larger binoculars that use higher magnification. BigBinoculars.com is the only binocular retailer in the USA that routinely sells general purpose and astronomy binoculars that support extreme magnification (as high as 150x.) At these magnifications, collimation must be as perfect as mechanically possible, regardless of IPD setting. We test and tune at that same level of perfection each and every binocular we sell- whether it's a $3000 Miyauchi Saturn III with 150x eyepieces, an Oberwerk BT-100 using Vixen 7mm's (90x), or a $99 Oberwerk 8x56.
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