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What you need to know before you buy binoculars


What Do The Numbers On Binoculars Mean?

Magnification and Objective Lens Size

All binoculars use the same method to define their technical specs. So for example a pair of binoculars may be described as 10 x 50. So what does this mean?

The “10” refers to the magnification power of the binoculars. This basically means that objects viewed will appear to be 10 times (10x) closer than when viewed with the bare eye.

The second number (in this case “50”) indicates the diameter of the objective lens (the light gathering lens at the front of the binoculars); this is measured in millimeters. Note that the diameter of the lens is directly related to the size of the binoculars—the larger the objective lens, the larger (and normally brighter) the size of the binoculars.

Field Of View

When you look through your binoculars, the widest dimension you can see is known as the field of view. So for example a field of view of 390´ (feet) indicates that the width of the image you see is 390 feet at a distance of 1000 yards. Some binoculars feature unique lenses to provide a wide field that is greater than normally seen through binoculars of the same magnification (awide-field pair of binoculars is better for observing fast-moving objects, such as herds of racing animals like antelope). You should also be aware that the field of view diminishes as magnification increases.

Exit Pupil

You can see the exit pupil when looking through the eyepiece of your binoculars held at arm’s length—that little stab of light you see in the ocular lens, is the exit pupil. So what’s its relevance?

The diameter of the exit pupil determines how much light is transmitted to your eye…in other words, it determines how much you get to see (this is mostly of significance in low-light conditions).

The exit pupil is determined by dividing the objective lens size by the magnification. So for binoculars with specs. of 10 x 50 or 7 x 35 the exit pupil size would be 5mm. this is how it’s determined:

Exit Pupil=objective lens size / magnification

Exit Pupil=50/10=5mm


Exit Pupil=35/7=5mm

The diameter of the exit pupil does not matter much if there’s enough ambient light, since the pupils of your eyes are generally smaller than the exit pupil size of your binoculars. However with dimming of ambient light, the pupils of your eye adapt by enlarging. This means that if the exit pupil size of your binoculars is comparatively small, in low light conditions this is going to be a significant restrictive factor. Consider the following example:

A pair of compact binoculars has the following specs: 8 x 25 i.e., magnification of 8 x and objective (front) lens of 25mm. As discussed above, we can easily determine the exit pupil size;

Exit Pupil Size=objective lens/magnification=25/8=3.1mm (approx.)

When ambient light gets low enough the pupils of your eyes adapt by increasing in size. If their size increase is greater than 3.1mm this means that size of your pupils is now greater than the exit pupil size of your binoculars…in simple terms this means that your binoculars aren’t transmitting enough light to your eye. Now consider if your binoculars had the following specs: 7 x 50

Exit Pupil Size=50/7=7.1mm (approx.)

7.1mm diameter is the same (slightly greater actually) aperture size of fully-dilated pupils of eyes in excellent condition (typically found in young people). With binoculars that possess similar specs. (7.1mm exit pupil size), the amount of light transmitted to your eyes is never an issue. You’ll always be able to see well in low light conditions.

Note that as we age the ability of our eyes to adapt to dim light is diminished. Our pupils no longer widen to the same extent as when we were younger. In fact the pupils of the elderly generally don’t dilate beyond 4mm—so as one ages, the exit pupil size requirement decreases.


This refers to the optical and mechanical alignment of binoculars. Cheap binoculars are often factory-shipped out of collimation. Good binoculars are carefully collimated, usually with laser precision. This translates into time and added cost at the manufacturing level—which translates into “more expensive” at your level (but it’s worth it in the long run).

Poorly collimated binoculars will give you eyestrain and headaches.

Twilight Performance

The Twilight Performance is a measure of the capability of binoculars in low conditions.

A quick way to determine the Twilight Performance is to multiply magnification with objective lens diameter. The higher the result the better the twilight performance


The old adage “you get what you pay for” is never truer than when applied to binoculars. If you skimp on price you most definitely know it (and regret it) sooner than later.

Eye Relief

The distance between your eye and the eyepiece is referred to as “eye relief”. Good eye relief is one of the important performance factors. Eye relief is especially important if you wear glasses. If you need to use binoculars while wearing spectacles or sunglasses look for models that provide at least 15mm of eye relief.


The objective lenses are found at the front of the binoculars. They gather light from whatever it is you’re looking at, and magnify the resulting image. This image at first, is upside down, and needs to be righted. This is the function of the prisms; they correct the orientation of the image so that by the time it gets to your eye it’s focused the right way up.

Types of Prisms

Porro Prisms Models–Porro prism models will give you the best optics for your dollar, but lack the durability and compactness of roof prism models. You can easily recognize them by their offset tubes–the objective lens and the occular lens are not in line. The porro design is generally optically superior to the roof prism design, especially when dealing with mid-priced binoculars. Porro prism models are easier to adjust for the spacing between your eyes because they have a single pivot between the two lens tubes of the binoculars.

Porro Prism Sub-Types–There’s something else you should take into consideration if you are going to buy a Porro prism model.

BAK-4 Prism Model–They are the best prism model type. They are designed with superior optical glass that results in clearer images. This type is what you should aim for.
BK-7 Prism Models–BK-7 prisms are usually found in lower priced models. They produce an inferior image and some manufacturers will not include this particular technical detail under the specification list because those of you in the know will be alerted to the fact this is an inferior design.

Roof Prism Models–are more compact. This is possible because of the way they are designed…they have straight tubes because the objective (front) lens is in line with the ocular (rear) lens. They commonly have two pivot points between the tubes, thus are harder to adjust for the spacing between your eyes. Roof prism models are usually more durable and compact, but you’ll need to spend more money to get good optical quality (or the equivalent optical quality of a Porro prism model with the same technical specifications).

Lens Coating

Light transmission through your binoculars is influenced by the use and type of anti-reflective coating on the binoculars lenses. How come? Because when anti-reflective coating is applied to the glass surface of binoculars it increases the amount of light transmitted to your eye. In simple terms what this means is the better the type of anti-reflective coating the sharper the resultant image.

Here are some of the terms you may come across associated with lens anti-reflective coating.

C: Some surfaces coated
FC: Other than plastic lenses, all surfaces coated
MC: Some surfaces coated with multiple layers
FMC: All glass surfaces coated with multiple layers

FMC–Fully Multi-Coated means that all air-to-glass surfaces have been multiple-layer coated which translates into the best image. This is the best quality, the downside being they are going to cost more.


For The Safari Traveller This Is What You Should Remember

Best avoid zoom binoculars
The theory seems appealing but the reality is very different. Most zoom models are virtually impossible to hold steady at higher magnifications and have very limited fields-of-view

Compact Models
Since you’re going to be on the move a lot and most likely doing most of your viewing in good daylight, you should probably settle for a good compact model–models with a magnification between 7 and 10. Probably the best all-rounder is a good pair of 7×35 model. They are bright enough to allow observation in low light conditions and portable enough not to be a weight burden.

Getting a pair of binoculars is a lifetime investment so try and buy the best possible model that you can. Remember, you get what you pay for!

Have A Great Safari!