A guide to binoculars

Over 30 years of keeping watch in dark woods at dusk and stalking on open moorland has shown us at ArdMoor the importance of investing in the right optics. The range of products available, their individual merits and purposes can make selecting the right binocular or riflescope a daunting task. So we’ve put together this comprehensive guide to help you understand which product is best suited to your planned activity. If you have any questions about our binoculars, rangefinders and telescopic sights online, please contact us. We will be happy to help!

Two Key Starting Points – Magnification and Objective Lens
With any set of binoculars or riflescope, the magnification and objective lens diameter give you the primary pieces of information you need to determine the lenses and scope of application. These values are stated by two figures e.g. 8×42 or 10×42 – the first figure states the binoculars’ magnification. The second details the objective lens diameter in millimetres.

Binoculars & Riflescopes – Magnification Explained
A small magnification gives a better field of view, sharper depth-of-field but with less detail. A large magnification gives a more detailed picture with a smaller field of view. Thus an optic with large magnification provides a depth of field that is not as sharp as one with small magnification.

A lot of telescopic sights have variable magnification which makes it possible to choose the right magnification to suit the terrain you are in, the light conditions you are operating in and the length of shot you anticipate having to make.

Objective Lens Size – What Does it Mean?
The objective lens diameter determines the amount of light that can enter the binoculars which, in turn, determines the light transmission efficiency. The larger the objective lens the greater the light transmission and the better the contrast particularly if you are in low light such as at dawn and dusk.

So How to Choose the Right Level of Magnification and Objective Lens?
The right level of magnification and objective lens diameter for you all depends on the nature of your planned activity.

Binoculars or Riflescopes for Stalking at Dawn or Dusk
For any type of hunting or stalking early or late in the day, you’ll need to choose binoculars or telescopic sights with a large objective lens diameter – aim for one of at least 35mm to give plenty of good light transmission.

Binoculars or Riflescopes for Hill Stalking and Extreme Hunting
For those who need to spot potential quarry at a longer range, choose a 7-8 or – better still – 10 x magnification with a relatively large objective diameter between 35mm & 45mm to provide better light transmission for richer contrasts.

Binoculars or Riflescopes for Deer Stalking
For deer stalking, there is a wide range of suitable binoculars, but have regard for the conditions and terrain you are going to be operating in. For red deer stalking on the open hill, you need a greater magnification, but light transmission is less important (although still a factor if you are likely to be out at dawn or dusk). For roe stalking in woods, you need less magnification as you’re operating at closer range, but the shade of the trees means you’ll need more light transmission. An 8×42 or 10×42 Binocular would be a good choice to give a good balance for most situations.

Binoculars for All Round Use
Whether it’s an early morning birdwatching or a day at the races, a good general purpose set of binoculars should give you the right balance between light transmission, depth of field and portability. Aim for a reasonably lightweight, compact pair of binoculars with somewhere between 7-10x magnification and an objective lens diameter of up to 42mm.

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The A to Z to Binoculars & Riflescopes Terminology
Coatings are the microscopic layer of refractive material such as magnesium fluoride which are applied to the lenses to improve light transmission and minimise reflections.

Exit Pupil
The Exit Pupil describes the amount of light that reaches your eye when holding the binoculars or telescope at arm’s length i.e. it’s the small circular image that’s visible in each eyepiece lens. The Exit Pupil is measured in millimetres and is found by dividing the objective diameter by the magnification eg. 10×42 = 4.2mm.

The average adult eye dilates to 7mm in extremely low light conditions. For binoculars or telescopic sights to deliver their full light gathering capability, the Exit Pupil must be equal to or greater than the diameter of your pupil at any given moment. Thus 2-3mm is sufficient in normal daylight but, for low light conditions it should be at least 5mm.

This is the actual width of the sight picture provided by your binoculars or riflescope at a distance of 1000m. Field-of-View can also be expressed in degrees. One degree is equal to 17.5m so if a Field-of-View is 8 degrees then the actual width of you view at 1000m would be 140m (8×17.5).

Magnification is a measure of how many times better you can see an object through your binoculars than with the naked eye. In the standard specifications for a binocular or telescope model, magnification is reflected by the first number you see. For a 10x magnification you would see an object that is 1000m away as if it were 100m away. For an 8x magnification that same object would appear as if it were 125m away.

Objective Lens Diameter
The diameter of the objective lens is stated in millimetres and the size determines how much light can enter the binocular or telescope. It combines with magnification, exit pupil, the type of coating and the type of prism to determine overall light transmission.

Twilight Performance
The Twilight Performance gives a basic evaluation of low light performance. It should be remembered that it is a mathematical equation and doesn’t take into consideration glass quality, number of lenses, coatings etc. It is calculated by first multiplying the magnification by the objective lens diameter and then finding the square root of the result.

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