A Guide to Buying Binoculars

Understanding Binocular Technology and How to Select What You Need

Buying binoculars means considering many different options and tradeoffs.
Buying binoculars means considering many different options and tradeoffs. Photo © Daniel H. Bailey / Getty Images.

I was recently looking to buy a new pair of binoculars and when I first started shopping around, I was bewildered by the vast number of makes and models available these days. Since I received my first pair of binoculars as a gift more than twenty years ago, I lacked experience shopping for the gadgets myself. I knew I had a lot to learn if I was going to invest my money wisely.

I started by scouring the web for insights and product reviews.

The good news is that there is a wealth of superb information out there, from tips and technical details to product reviews and side-by-side comparisons. The bad news is there is so much information available you may end up more confused after reading it all than you were to begin with.

To temper my own confusion, I decided to put together a summary of the information I gathered from various sources (you'll find a complete list of my sources at the end of this article, each is well worth checking out).

It's All About Trade-Offs

A recurring theme in binocular design is that there are trade-offs. You simply can't get everything all the time with one set of binos. For instance, if you opt for high magnification, you forfeit some of your field of vision and a bit of brightness. If you opt for low magnification, you forfeit detail but gain field of vision and brightness. We'll get to the nitty gritty of these trade-offs later, but for now, remember that when it comes to binoculars, to gain something usually means you have to give up something else.

Put Yourself First When Buying Binos

When selecting binoculars, it's easy to get bogged down with prism type, magnification, field of vision and an assortment of technical specifications. But remember to always put yourself first when buying binoculars. Think in terms of what you want out of your binoculars.

Let that determine what you need in them. For instance, you should focus on:

  • Image Quality
  • Viewing Conditions
  • Comfort and Ease of Use
  • Price

Don't insist on having 10x50s if they are too heavy for you to pick up. Or don't get obsessed with roof prisms because they are the latest design. Focus instead on the image quality, level of comfort, and the price you seek, then decide on which set of features meet your needs.

Binocular Basics

Having said that, you should still be familiar with the nuts and bolts of binocular technology. The basic factors to consider include:

  • Prism Type
  • Magnification
  • Field of Vision
  • Minimum Close-Focus Distance
  • Eye Relief
  • Exit Pupil

These factors differ from one model of binoculars to the next and influence what you get out of your binoculars in terms of image quality, viewing conditions, comfort, ease of use, and price.

Prism Type

If it weren't for the prisms in binoculars, you would see an upside-down, mirror-reflection of the world through your binoculars. Prisms sort out the image so it reaches the eye in proper orientation without such surprises. There are several types of prisms that accomplish this:

  • Roof prism - straight profile, eyepieces in line with objective lenses, fairly new design.
  • Porro prism - classic design, eyepieces set closer together than the objective lenses.
  • Reverse porro prism - eyepieces set wider than the objective lenses, common in compact binoculars.

Some trade-offs to consider when selecting prism type are:

  • Porro prisms lose little light or image quality.
  • Porro prisms yield better performance and image quality when compared to roof prisms of the same price (it simply costs less to produce a Porro prism).
  • Roof prisms enable a binocular design that is easier to hold and use. Don't underestimate the comfort factor.
  • Roof prisms are more complex and alignment is more critical than in Porro prisms. This requires roof prism binoculars to be very well made to reduce the chances of misalignment of the prisms.
  • Roof prisms lose more light than Porro prisms and require special coatings to compensate (this in turn inflates the price tag).
  • Roof prisms split the image and when it rejoins it is slightly out of phase, leading to decreased image quality. To compensate, roof prisms must have phase correction technology that eliminates this problem (again, this inflates the price tag).
  • Reverse Porro prisms are usually only used for compact binoculars.

Magnification

Binoculars are described by two numbers, for example 7x35, 8x40, or 10x50. The first number is the magnification (or power) and the second is the diameter of the objective lens (in millimeters). Binoculars used most commonly for bird and wildlife watching are 7x, 8x, or 10x.

Some trade-offs to consider when selecting magnification are:

  • As magnification increases, field of vision decreases and it is more difficult to locate a subject through the binoculars.
  • As magnification increases, image detail increases so field markings are easier to see.
  • As magnification increases, the effects of a shaky grip on the binoculars are exagerated.
  • As magnification increases, the amount of light transmitted to the eye decreases so you may not be able to identify subjects as the sun drops below the horizon.
  • As magnification increases, the minimum close-focus distance increases. This means if you intend to watch butterflies at close range, you may want to consider a lower magnification.

Field of Vision

Field of vision describes the width of the viewing area visible through your binoculars. Field of vision is influenced by numerous variables in binocular design but magnification and objective lens diameter are the most important factors.

Higher magnifications reduce the field of vision for equal objective lens diameters. Larger objective lenses yield larger fields of view for equal magnifications. The field of vision is normally listed with the binocular specifications as the number of degrees or feet you would see in the viewing area at 1000 yards.

Some trade-offs that affect field of vision are:

  • A larger diameter objective lens will give you a larger field of vision, all other factors being equal.
  • A larger diameter objective lens will result in a heavier set of binoculars.
  • A larger diameter objective lens results in a brighter image.

Minimum Close-Focus Distance

The minimum close-focus distance is a measure of the minimum distance you must be from an object before you can bring that object into focus through the binoculars. If you plan to watch butterflies, it is often useful to be able to focus on objects that are quite close, for example.

Some trade-offs that affect minimum close-focus distance are:

  • A closer minimum close-focus distance usually means forfieting magnification.
  • A closer minimum close-focus distance often means an reduction in overall image clarity.

    Eye Relief

    Eye relief refers to the optimal distance between your eye and the ocular lens of the binocular. Eye cups are designed to hold the binoculars at the correct distance from your eyes but this only works if you don't have to wear glasses. Without glasses, you don't have to worry about eye relief since the eye cups are designed to give you the proper fit. But if you wear glasses, eye relief becomes an important factor to check before buying binoculars.

    If you wear glasses, then you will need to turn down the eye cups on your binoculars. The distance your eye glasses sit from your eye must closely approximate the eye relief distance of the binocular design to offer you a good image.

    Some trade-offs to consider about eye relief distance are:

    • If the eye relief distance your binoculars are designed to accomodate is less than what your glasses will allow, you are more likely to experience tunnel vision when looking through the binoculars.
    • If the eye relief distance your binoculars are designed to accomodate is greater than what your glasses will allow, you may end up holding the binoculars away from your glasses which will be uncomfortable and potentially unsteady.

    Exit Pupil

    The exit pupil is the shaft of light transmitted through the binoculars to the eye. The exit pupil is an indicator of how well the binoculars will perform in low light conditions. Larger exit pupils transmit more light and thus a brighter image.

    Exit pupils also reveal the quality of the image. If you hold your binoculars a few feet from your eye and view the exit pupil, it will have sharp edges if the binos have high image quality.

    Finally, exit pupils provide you with a method of checking the alignment of a pair of binoculars. If you positioning the binoculars on a flat surface and point them at a horizontal line (such as a window ledge or shelf), you can then check that the line is even between the exit pupils.

    The Decision Making Process

    The following describes the basic process of selecting binoculars. These steps are based on the recommendations of the Cornell Lab of Ornithology.

    1. Determine how you will use your binoculars. Think about the ways you plan to use your binos. Will you watch birds? Do you plan to watch butterflies? How close do you plan to get to your subject? Do you need to see a high level of detail?

    Do you need to see a large area of landscape? Understanding how you intend to use your binoculars is necessary before you can proceed with steps 2 and 3.

    2. Determine the magnification you need: 7x, 8x, or 10x. This decision should be easier if you know exactly how you intend to use your binoculars. It's also down to a bit of personal preference. Whenever possible, test drive binoculars with different magnifications to see if they're right for you. Take a peak through a fellow birder's binos or visit an optics store and ask to peek through a range of magnifications.

    3. Identify any features that are important to you. Do you want a wide field of vision? Do you need lightweight binoculars? Do you have glasses and need adjustable eye cups to obtain the appropriate eye relief? Do you plan to bird watch at dusk or in dim lighting conditions? Once you know what features are important to you, you can then weigh any trade-offs you'll need to make.

    5. Determine what you're willing to spend. The money part, though painful, is simply another trade-off. Do you want to hand over extra cash to obtain more features? Do you want to wait and save some money until you can afford a certain pair of binoculars?

    6. Narrow Your Search and Make Your Purchase. Make a shortlist of binoculars you're interested in.

    Be sure to read some reviews and to test drive the models on your shortlist before you make your purchase. After that, enjoy!

    What to Do With Your Old Binoculars

    In his article 'The Age of Binoculars', Kenneth V. Rosenberg, director of the Conservation Science program at the Lab of Ornithology, brings up a great idea for those of us replacing an old pair of binoculars:

    "I ask you to consider the fate of your old optics. As birding and bird-conservation efforts flourish worldwide, ornithologists and birding guides throughout Latin America and the Caribbean often lack the means to purchase the basic tools of their trade."

    If you would like to ensure your retired binoculars won't gather dust and will instead be put to good birding use, you can donate your binos via the American Birding Association’s Birders’ Exchange at www.americanbirding.org/bex or Optics for the Tropics at www.opticsforthetropics.org.

    Postscript

    I'd like to note that I've not yet purchased my new set of binoculars. So far, I've made it through Step 5. Determine what you're willing to spend. After weighing my options, I've decided to save up some more cash before I make my purchase. I'd prefer to wait and purchase a pair of binoculars that will last me decades than to purchase something that I will want to replace a few years down the road.

    References

    Birding Binoculars and How They Work (Birdwatching.com)

    The Age of Binoculars (Cornell Lab of Ornithology)

    Binocular Prisms (Shaw Creek Bird Supply)

    Desperately Seeking Binos (Cornell Lab of Ornithology)

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    Klappenbach, Laura. "A Guide to Buying Binoculars." ThoughtCo, Aug. 11, 2016, thoughtco.com/guide-to-buying-binoculars-130243. Klappenbach, Laura. (2016, August 11). A Guide to Buying Binoculars. Retrieved from https://www.thoughtco.com/guide-to-buying-binoculars-130243 Klappenbach, Laura. "A Guide to Buying Binoculars." ThoughtCo. https://www.thoughtco.com/guide-to-buying-binoculars-130243 (accessed December 12, 2017).