Tips for Purchasing Binoculars



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Date: 7-18-2001
Abstract: Some tips for people looking to buy binoculars

Binoculars are perhaps the best of all of the observing tools.  They are simple to use, easily transportable, and extremely reasonable.  I don’t know of any serious astronomers who do not own a decent pair of binoculars.

The problem is like most other astronomy equipment the choices can be sometimes overwhelming for new astronomers.  Everyone seems to have an opinion about what to get and how much to spend.  Hopefully I can help you make the first major decision of your astronomical career.

Why Are Binoculars So Important

Binoculars serve many different uses while observing.  Most people use them as quick spotting instruments before they move their main scope.  The right side up view allows you to easily orient yourself before looking through the backward or mirror image view in your telescope.  Some people also use binoculars as their primary observing instrument.  For example here in the winter it can get quite cold and there are relatively few cloudless nights.  During that time of year I rely almost exclusively on my binoculars for making observations.  It is very rare when I set up my large telescope during the winter months.

Binoculars are also multi use instruments.  Almost everyone has a use for binoculars.  So you will often find yourself using them for more than astronomy.  So you can consider your investment in binoculars to be more than an investment in astronomy.

Types of Binoculars

There are tow major types of Binoculars.  They look slightly different and are classed by the prism type that they use.  They are the Porro Prism Binocular and the Roof Prism Binocular.

The Porro Prism Binoculars are the most common type of binoculars.  They consist of two lenses in the front with prisms that bend the light into the eyepieces in the back.  This type of binocular is said to be a better performer for astronomy.

The Roof Prism Binoculars are generally more expensive and much more rare than the other type.  They look quite simply like two telescope mounted next to each other.  In essence that is what they are with a prism system to correct the image.

Lens Diameter (Aperture)

Like telescopes one of the most important features of an astronomical binocular is the diameter of the front lens system.  This determines how much light will be collected and in turn determines how bright faint objects will appear.  The most common size for astronomical binocular is about 50mm.  Some people do prefer smaller but anything below 42mm is not terribly useful for astronomy.  50mm binoculars are the perfect size for collecting a good amount of light while still being light enough to actually hold in your hands.  Anything above 60mm is an excellent astronomical tool but will probably require a separate mount due to their weight.  So if you plan on using your binoculars as quick look instruments stay in the 50mm range (+/- 10mm) and if you want to use them as a primary observing instrument you should look at 60mm and above.

As a side not the aperture (size) of the lens is the second number usually listed on the binocular.  Such as 10 x 50 or 6 x 42.  Those are 50mm and 42mm lenses respectively.


The second important aspect of the binocular is the magnification.  Binoculars tend to range from about 6x to 20x magnification.  Most hand held binoculars are below 10x.  Like telescopes the magnification determines how much closer something will appear.  Also like telescopes high magnifications are usually problematic.  High magnification binoculars (10x and above) will usually have to be mounted to be useful.  The magnification also determines the field of view.  One of the reasons people buy a pair of binoculars is to get a nice wide view of what they are looking at.  Lower magnification binoculars tend to have wider views, ie: they show more space.

But there is a catch.  The magnification that is the best for you is really determined by where you live.  As a general rule the darker the sky the lower the magnification your eyes require.  For urban and suburban people it is generally recommended that you purchase binoculars in the 8x to 12x range – 10x being the best.  If you happen to live under dark skies or travel to them often anything from 6x to about 9x will give nice bright views – 7x being the most common.

The magnification is the first number listed on the binocular.  Such as 10 x 50 and 6 x 42.  Those are 10x and 6x respectively.

Prism Types

Although we have already talked about the design of the prisms there are a few different types of glass that they use in them.  The two major glass types are BAK-4 and BK-7.  Forget about what they stand for all you have to know is that the BAK-4 prisms tend to be brighter and better for astronomical viewing.  Given the choice go with BAK-4 prisms even if they cost a little bit more.


Optical coatings are applied to the lenses and eyepieces of most binoculars on the market today.  These coatings stop light from reflecting off of the lens and allow more light to reach your eyes.  Although some companies use proprietary names for the amount of coatings they apply the most common names are.

Ruby Coated – A gimmick.  These are usually sold as “night vision” binoculars.  They really do nothing more than turn the view a greenish, night vision like color.  They are very bad for astronomy because they actually dim the view.

Coated – This is the lowest level of coating available.  This consists of usually one layer of coatings on the front lens element and the eyepiece.  When viewed from an angle the lens will have a very faint bluish tint.

Fully Coated – FC – Every glass to air surface has one layer of coatings.  These are usually the standard in lower priced binoculars.

Multi Coated – MC – This is the most common coating system for affordable (under $100) binoculars.  Quality wise they are quite a step up from simple coatings.  This usually means that every glass to air surface is at least coated, and some of them are coated with multiple layers (usually the front lens and eyepiece lens).  When viewed at an angle these lenses show a significant blue or green tint.

Fully Multi Coated – FMC – SMC – The highest level of coating generally available.  Basically every glass to air surface has multiple layers of coatings.  When viewed at an angle these lenses show many different colored reflections.

All things equal the coating in your binoculars will make quite a difference.  I would not recommend any binoculars with less than a Multi Coated ranking.  If you are serious you may want to consider only Fully Multi Coated binoculars.


As you can see binoculars are quite complicated little instruments.  The following table may help you pick the right pair for you observing needs.

Observing Location Below $100 $100 - 150 Above 150
Urban 10x50FC/MC 10x50MC/FMC


Suburban 10x50MC 10x50MC/FMC 10x50FMC
Rural 7x50MC 7x50MC/FMC 7x50FMC

Submitted by Curt Irwin - - Grand Rapids, MI

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