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Complete Guide to Binocular Sizes & Uses
Complete Guide to Binocular Sizes & Uses

Perhaps you've had a telescope for many years now. Maybe you've observed through almost every size imaginable and your optics fleet contains everything just short of the Hubble. You've logged time under genuinely dark skies and paid your dues chasing down some of the most difficult and distant deep space objects that have ever graced a star chart. Now what? Maybe it's time to get back to the basics, and challenge your optical explorations with a good pair of backyard binoculars.

Astronomy is most enjoyable when it is a relaxed experience. Sure, it's exciting to travel to a remote locale with sophisticated equipment, but a satisfying experience doesn't have to mean entire nights spent alienated from your family or totally missing out on television. A good pair of binoculars can be the difference between five or ten minutes of sweet stellar insights, and none at all. Whether you're new to the game and interested in practicing serious binocular astronomy or just want a casual cosmic close-up on a working night, these "twin telescopes" are both convenient (portable) and affordable.

Orion 10x50 UltraView Wide-Angle Binocular

Orion 10x50 UltraView Wide-Angle Binocular

How Binoculars Work:
If you're just beginning, then learning more about how binoculars operate and what style of binoculars work best for astronomy applications would be a great place for us to start. Binoculars are both technical and simple at the same time. They consist of an objective lens (the large lens at the far end of the binocular), the ocular lens (the eyepiece) and a prism (a light reflecting, triangular sectioned block of glass with polished edges). The prism folds the path of the light and allows the body to be far shorter than a telescope. It also flips the image around so it doesn't look upside-down. The traditional Z-shaped porro prism design is well suited to astronomy and consists of two joined right-angled prisms which reflect the light path 3 times. The straight barreled roof prism models are more compact and far more technical. The light path is longer, folding 4 times. These mechanics require a huge amount of quality control to equal the performance and are better suited to terrestrial subjects.

Understanding the Optics: Objective Lens Sizes
Now, let's get a little more technical: All binoculars have a pair of numbers associated with them - the magnifying power times (X) the objective lens size. For example, a popular ratio is 7X35. For astronomical applications, these two numbers play an important role in determining the exit pupil - the amount of light the human eye can accept (5-7mm depending on age from older to younger). By dividing the objective lens (or aperture) size by the magnifying power you can determine a pair of binoculars exit pupil.

Like a telescope, the larger the aperture, the more light gathering power - increasing proportionately in bulk and weight. Achieving stereoscopic views of the night sky through big binoculars is an incredible, dimensional experience and one quite worthy of a mount and tripod! As you journey through the binocular department, go armed with the knowledge of how to choose your binoculars lens size. Why does the binocular lens size matter? Because binoculars truly are a twin set of refracting telescopes, the size of the objective (or primary) lens is referred to as the aperture. Just as with a telescope, the aperture is the light gathering source and this plays a key role in the applications binoculars are suited for. Theoretically, more aperture means brighter and better resolved images - yet the size and bulk increases proportionately. To be happiest with your choice, you must ask yourself what you'll be viewing most often with your new binoculars. Let's take a look at some general uses for astronomy binoculars by their aperture.

What Size is Best for What?
Binoculars with a lens size of less than 30mm, such as 5X25 or 5X30, are small and very portable. The compact models can fit easily into a pocket or backpack and are very convenient for a quick look at well-lit situations, making them ideal for bird watching and nature walks. In this size range, low magnifications are necessary to keep the image bright. Compact models are also great binoculars for very small children. If you're interested in choosing binoculars for a child, any of these models are very acceptable - just keep in mind a few considerations: Children are naturally curious, so limiting them to only small binoculars may take away some of the joy of learning. After all, imagine the thrill of watching a raccoon in its natural habitat at sundown? Or following a comet! Choose binoculars for a child by the size they can handle, whether the model will fold correctly to fit their interpupillary size, and durability. Older children are quite capable of using adult-sized models and are naturals with tripod and monopod arrangements. For less than the price of most toys, you can put a set of quality optics into their hands and open the door to learning. Children as young as 3 or 4 years old can handle 5X30 models easily and enjoy wildlife and stargazing both!

Binocular aperture of up to 40mm is a great mid-range size that can be used by almost everyone for multiple applications. In this range, higher magnification becomes a little more practical. For those who enjoy stargazing, this is an entry level aperture that is very acceptable to study the Moon and brighter deep sky objects and they make wonderful binoculars for older children.

 

Orion's Savannah Pro ED 8x42 are a great choice for birders and wildlife enthusiasts.

Orion's Savannah Pro ED 8x42 are a great choice for birders and wildlife enthusiasts.

 

Binoculars up to 50-60mm in lens size are also considered mid-range, but far heavier. Again, increasing the objective lens size means brighter images in low light situations - but these models are a bit more bulky. They are very well suited to astronomy, but the larger models may require a support (tripod, monopod, car window mount) for extended viewing. Capable of much higher magnification, these larger binocular models will seriously help to pick up distant, dimmer subjects such as views of distant nebulae, galaxies and star clusters. The 50mm size is fantastic for older children who are ready for more expensive optics, but there are drawbacks.

The 50-60mm binoculars are pushing the maximum amount of weight that can be held comfortably by the user for long periods without the assistance of a tripod, but don't rule them out. Available in a wide range of magnifications, these models are for serious study and will give crisp, bright images. Examples are the new Orion E-Series 10x50 Waterproof Binoculars, or the 10x50 UltraView Wide-Angle Binoculars. Delicate star clusters, bright galaxies, the Moon and planets are easily distinguishable in this aperture size. These models make for great leave-in-the-car telescope so you always have optics at hand. For teens who are interested in astronomy, binoculars make an incredible "First Telescope." Considering a model in this size will allow for most types of astronomical viewing and with care will last through a lifetime of use.

Binoculars any larger than 50-60mm are some serious aperture. These are the perfect size allowing for bright images at high magnification. For astronomy applications, binoculars with equations like 15X70 or 20X80 are definitely going to open a whole new vista to your observing nights. The wide field of view allows for a panoramic look at the heavens, including extended comet tails, large open clusters such as Collinder Objects, starry fields around galaxies, nebulae and more. For binoculars in this size range, a tripod is strongly suggested to get the most from these instruments. If you have never experienced binocular astronomy, you'll be thrilled at how easy objects are to locate and the speed and comfort at which you can observe. A whole new experience is waiting for you!

 

Orion 20x80 Astronomy Binoculars.

Orion 20x80 Astronomy Binoculars.

 

When choosing binoculars for astronomy, just keep in mind that all binoculars are expressed in two equations: magnifying power X the objective lens size. So far we have only looked at the objective lens size. Like a telescope, the larger the aperture, the more light gathering power - increasing proportionately in bulk and weight. Stereoscopic views of the night sky through big binoculars is an incredible, dimensional experience, but for astronomical applications, these two numbers to play an important role in determining the exit pupil, or the amount of light the human eye can accept. By dividing the objective lens (or aperture) size by the magnifying power you can determine a pair of binoculars' exit pupil. Let's take a look at why that's important.

Understanding the Optics: Magnification
How do binoculars magnify? What's the best magnification to use? What magnifying power do I choose for astronomy? Where do I learn about what magnifying power is best in binoculars? Because binoculars are a set of twin refracting telescopes meant to be used by both eyes simultaneously, we need to understand how our eyes function. All human eyes are unique, so we need to take a few things into consideration when looking at the astronomy binocular magnification equation. By dividing the objective lens (or aperture) size by the magnifying power you can determine a pair of binoculars' exit pupil and match it to your eyes. During the daylight, the human eye has about 2mm of exit pupil - which makes high magnification practical. In low light or stargazing, the exit pupil needs to be more around 5 to be usable.

While it would be tempting to use as much magnification as possible, all binoculars (and the human eye) have practical limits. You must consider eye relief, or the amount of distance your eye must be away from the secondary lens to achieve crisp focus. Many high "powered" binoculars do not have enough outward travel for eye glass wearers to come to focus without your glasses. Anything less than 9mm eye relief will make for some very uncomfortable viewing. If you wear eyeglasses to correct astigmatism, you may wish to leave your glasses on while using binoculars, so look for models which carry about 15mm eye relief or larger.

Now, let's talk about what you see! If you look through binoculars of two widely different magnifying powers at the same object, you'll see you have the choice of a small, bright, crisp image or a big, blurry, dimmer image - but why? Binoculars can only gather a fixed amount of light determined by their aperture, or lens size. When using high magnification, you're only spreading the same light over a larger area and even the best binoculars can only deliver a certain amount of detail. Being able to steady the view also plays a critical role. At maximum magnification, any movement will be exaggerated in the viewing field. For example, seeing craters on the Moon is a tremendous experience - if only you could hold the view still long enough to identify which one it is! Magnification also decreases the amount of light that reaches the eye. For these reasons, we must consider the next step: choosing the binocular magnification carefully.

Magnifications for Birding to Astronomy
Binoculars with 7X magnifying power or less, such as 7X35, not only deliver long eye relief, but also allow for variable eye relief that is customizable to the user's own eyes and eyeglasses. Better models have a central focus mechanism with a right eye diopter control to correct for normal right/left eye vision imbalance. This magnification range is great for most astronomy applications. Low power means less "shake" is noticed. Binoculars with 8X or 9X magnification also offer long eye relief, and allow comfort for eyeglass wearers as well as those with uncorrected vision. With just a bit more magnification, they become ideal for astronomy. Binoculars 10 x 50 magnifying power are a category of their own. They are at the edge of multipurpose eye relief and magnifying power at this level is excellent across all subject matter. However, larger aperture is recommended for locating faint astronomy subjects.

Binoculars with 12-15X magnifying power offer almost telescopic views. In astronomy applications, aperture with high magnification is a must to deliver bright images. Some models are extremely well suited to binocular astronomy with a generous exit pupil and aperture combined.

 

Orion BT100 Premium Binocular Telescope with 22x magnification.

Orion BT100 Premium Binocular Telescope with 22x magnification.

 

Binoculars with 16X magnification and higher are on the outside edge of high magnification and are barely within hand-held capabilities. They are truly designed exclusively as mounted astronomical binoculars. Most have excellent eye relief, but when combined with aperture size, a tripod or monopod is suggested for steady viewing.

Keep in mind little things that might be good for your applications, like rubber-coated binoculars for children who bang them around more, or fog-proof lenses if you live in a high humidity area. Cases, lens caps and neck straps are important, too. While so much information on binoculars may seem a little confusing at first, just a little study will take you on your way to discovering astronomy binoculars that are perfect for you. As you use a new pair of binoculars, you'll find that you adapt to them, and can be quite skilled at locating objects with ease. Once you feel comfortable with your binoculars, see if you can see any of the objects in this list: Orion's Top 25 Binocular Objects. Clear skies!

Details
Date Taken: 02/13/2014
Author: Tammy Plotner
Category: Equipment

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