How to Choose Binoculars

This is a work in progress.  Please check back soon, we are updating this regularly.

Welcome to the wonderful world of binoculars.  Whether you're exploring the night sky, seeking out interesting nature, taking in an event, or require some assisted vision, binoculars are an excellent tool that no observer should be without.

Binoculars are available with a wide variety of specifications, suitable for a wide variety of activities.  Selecting the optimal pair is crucial to your enjoyment.  Let's jump right into the specifications.

Porro Prism vs. Roof Prism

The two main styles of binoculars are Porro and Roof.

Porro prism binoculars utilize two right-angle prisms per side to correct the orientation of the image.  This style generally has a wider body format, with exception of their more compact reverse Porro cousins.  Wider-spaced lenses lead to a sense of greater depth in the image.  They are simpler to manufacture, and therefore tend to be the less expensive models.

Roof prism binoculars utilize a more complicated set of prisms to correct the orientation.  The use of special phase-correction coatings add to that cost.  Due to the objective and eyepiece lenses being inline, they tend to have a more slim-line format.  Depending on the method of focusing used, these often have better close focus ability. 




In the above example, 8x is the magnification factor; how large do objects appear in the binocular.

Your first thought is probably that a bigger first number is better, right?  The answer depends on a number of factors.

  • the ability to hold the binocular steady
  • field of view requirements
  • brightness requirements

In addition to making objects appear larger, magnification also enlarges the user's movements.  Some people are steadier than others and can use higher magnifications, but eventually a magnification will be reached where the image becomes unstable.

People often select binoculars while at rest, but what if you're active?  During a long walk, with your heart pumping and adrenaline coursing through your body, that binocular your carefully selected suddenly produces a jiggly image that's difficult for your eyes to track.  Your activity level should also dictate the magnification you chose.

One way to avoid the jiggles with high-powered binoculars is to mount them to a tripod.  Virtually all decent models on the market today are tripod-mountable.  Electronic image stabilized binoculars also exist for when a tripod is too cumbersome.

Magnification also affects brightness.  The lower the magnification, the more concentrated the light is, resulting in a brighter image.  Conversely, the higher the magnification, the more spread-out the light becomes, leading to a dimmer image.




Referring to the above example again, the second number represents the diameter of the objective lens in millimetres.  The objective lens is the lens closest to the object you wish to observe.

The objective lens plays three crucial roles:

  • light gathering
  • resolution
  • overall physical size of the binocular

Larger objective lenses collect more light, leading to a brighter image.  Resolution is improved, so you can discern finer details in the image.


7.4°     388ft @ 1000yds

Field of View

The field of view represents how wide you can see.  Wider fields of view are often more advantageous, as it makes locating subjects easier with less aiming around, and let you see more of the action.

Field of view is expressed in one of three measurements.

  • Degrees
  • Feet of width at 1000 yards distance
  • Metres of width at 1000 metres distance

Degrees tends to be easier to visualize.

You can convert degrees to ft@1000yds simply by multiplying the degrees by 52.5, since 1 degree = 52.5ft@1000yds.

Or convert degrees to m@1000m simply by multiplying the degrees by 17.45, since 1 degree = 17.45m@1000m.

A binocular showing a wide field of view doesn't necessarily provide a sharp and low distortion view across the entire field.  Often the opposite is true, unless the manufacturer put a lot of effort into correcting the field.  A perfect example of this is the Swarovski NL Pure series of binoculars, which have outstanding sharpness, aberration and distortion control over the entire field.


Close Focus

Imagine observing a bird feeder 3 metres from your kitchen window.  Having a binocular with good close focus means not necessarily having to move away from the window in order to focus clearly.  There will be lots of instances where nature will get up-close to you.


Exit Pupil

The exit pupil is a projected image of the objective lens, extending out the eyepiece.  It's calculated by the simple formula:

EP = D / M

Where D is the objective lens diameter, and M is the magnification.

This is important because our eyes also have a pupil, with a varying diameter depending on how much light enters it, our age, etc.  Pupils dilate when our eyes desire more light, and contract to reduce light.

When observing in low-light conditions, binoculars with larger exit pupils are more desirable, because they direct more light into the eye, however they tend to be large and bulky.

Observing in strong lighting conditions forces our pupils to contract, and the light from larger exit pupils is wasted since it cannot enter the eye.  That also means under these conditions, binoculars with exit pupils equal to or larger than your pupil diameter will appear to have the same brightness.

The largest exit pupil available is about 7.1mm, which matches our maximum pupil diameter at the best time of our lives.  As we age, our pupils will lose their ability to open as wide, often maxing out at 5 or 6mm.


Relative Brightness

To compare the relative brightness of different models, we perform a calculation similar to the exit pupil.

RB = (D / M)2

For example, the Relative Brightness of an 8x42 binocular is:

RB = (42 / 8)2
RB = 27.5


49 is the brightest on the relative brightness scale, because it is our pupil diameter squared.


Optical Coatings

Coatings can impact the image many different ways.  They can be used to reduce reflections, increase contrast, improve resolution, and reduce light loss.  They can also be used to tailor the overall colour balance of an image.

The main types of anti-reflection coatings are:

Fully Multicoated

All lens and prism surfaces receive multiple coatings, offering the highest degrees of light transmission.



Exterior-facing lens surfaces receive multiple coatings.  Interior-facing lens and prism surfaces receive a single coating.


Fully Coated

All lenses and prism surfaces receive a single coating.  This is usually found on really inexpensive binoculars, offering the lowest degrees of light transmission.


Roof prism binoculars make use of three other kinds of coatings:

Phase Correction (PC) Coated

Applied to the prisms, improve resolution and contrast by correcting for phase shift within the prism.  Inexpensive models don't use any PC coatings.


Metal-Vapourized High-Reflectivity Coated

Metals such as Aluminum or Silver are vapourized onto the reflection surfaces to reduce light loss.


Dielectric Coated

Non-metallic coatings using light interference applied to the reflection surfaces of the prisms to reduce the loss of light, thus increasing light throughput. 



It's important to find a binocular that is comfortable for your activity level, strength, and compatible with your physiology (eg; hand tremors).
Heavy models are probably not going to be a lot of fun to tote around all day in the field.
Light-weight models are practical.  However when coupled with high magnification, they become unusable hand-held, due to their lower inertia and the consequently jiggly views.



The vast majority of binoculars on the market today are waterproof.  It's something you don't think about, until you get caught in a rain shower, or tip over your canoe.  Waterproof binoculars tend to be a bit heavier than non-waterproof models.


Tripod Mountable

Many binoculars are tripod mountable, to increase stability, or to facilitate sharing.  They use a standard 1/4" 20tpi camera tripod thread, and usually require an L-bracket.  Some giant models do not require an L-bracket.


More to come.  Check back soon.