Choosing a binocular or scope - information


January 2024

The advantage of the full size binoculars (42mm's), is they give you great light gathering, and tend to be preferred by serius birders.

However, the 32mm mid-sized, mid-weight ranges are fabulous for takaing everywhere with you, being lighter weight and smaller.

We sell more 32mm binoculars nowadays than 42mm larger binos.

To look at, one pair of binoculars would appear to be much the same as another. Some might be smaller than others but, by and large, they’re made up of a pair of barrels with lenses at either end, attached to each other with a hinge in the middle.

Dig deeper, however, and you’ll discover there’s a whole world of features and specifications that determine how good a pair of binoculars are, and the sort of spotting activities they’re suited to.

You’ll probably have noticed if you’ve ever shopped for or used a pair of binoculars, that they all have a pair of numbers, like 8 x 42, or 10 x 25, written on them.



The magnification specifies the factor by which an object appears to be closer in comparison with the actual distance. The higher the magnification, the closer the object seems to be. However, a higher magnification also means a smaller field of view. Check the precise product name as the number in front of the ‘x’ specifies the magnification. For example, 10x42 is a device with 10x magnification. 

The first number designates the magnification level: an 8x pair will enlarge your subject eight times while a 10x pair will make it ten times bigger. The second number tells you how large the objective lenses (the big ones on the end) are in millimetres.

The most important number is the magnification level. You might think here that bigger is better, right? Well, that’s not strictly true. While a 10x or 12x magnification will allow you see things further away that bit closer, higher magnifications such as these can bring their own down sides. At higher levels of magnification, it's hard to hold binoculars steady enough to see a stable image. A higher magnification also usually means a narrower field of view. Field of view (or FOV), incidentally, is another number on the binocular somewhere. It's normally expressed in degrees, and it refers to how much you can see from left to right when you’re peering through them.

The following information aims to give you a basic understanding of how binoculars work, the different specifications available and what they mean, plus some points to help you choose the best instrument suited to you.

Specification Example: 8x42. The number ‘8’ denotes the magnification and means an object appears to be 1/8th of its actual distance away. Using this rule an object 40m distant appears 5m away. ‘42’ is the diameter of the objective lens (the large end) in mm through which light enters the binocular.

For birdwatching and nature-spotting, most people go for 8x. This gives you a reasonably broad field of view (usually around 7 to 8 degrees) and magnification and allows you to hand-hold easily without getting too much distracting shake. If you’re more of a stargazer and need binoculars for looking at the moon and constellations, you’ll want a higher magnification level, 12x and up, but you'll need to think about attaching them to a tripod.

Field of view

The field of view describes the size of the image section that can be seen through the optics. This is specified either in meters (width) at a distance of 1000 meters (m/1000m), or feet (width) at a distance of 1000 yards (ft/1000 yds), or as an angle (degrees). The higher the magnification, the smaller the field of view.
Binoculars have a large field of view, which means you can see a wide area. Spotting scopes have a higher magnification, which makes the field of view much smaller, but you can see more detail.

Objective lens diameter

The objective lens diameter specifies how much light can enter the optics. This makes it a key factor in an instrument’s performance, for example, in twilight. The bigger the objective lens diameter, the more light the objective lens can capture. The darker the surroundings, the larger the objective lens diameter needs to be. Check the precise product name as the number after the ‘x’ specifies the objective lens diameter in millimeters. For example, a device with the suffix 10x42 has an objective lens with a diameter of 42 mm.

Shortest focussing distance

The shortest focusing distance specifies how close an object needs to be to see it clearly with the optics. Between this value and infinity, it is possible to focus the image.

Roof prism binoculars

The popularity of roof prism binoculars is a result of this system being favoured in the development of instruments with user oriented features such as waterproofing, long eye relief and close focus.



What magnification? The higher the magnification relative to the objective lens size, the lower the brightness and the shallower the depth of focus (distance in focus at a single focus setting.) For general observation choose 7x or 8x. If you want 10x or more try them first as magnification amplifies hand-shake affecting image stability.

What objective lens size? The amount of light entering a binocular is related to the surface area of the objective lens ‘OG’. A 50mm OG will admit 2.5x the light of a 30mm OG. The amount of light reaching the eye is called the exit pupil diameter ‘EPD’ and its size can be found by dividing the OG diameter by the magnification. For example the EPD of an 8x32 = 4mm while the EPD of an 8x56 = 7mm. As a general rule your iris dilates 2-3mm in bright sunlight, up to 6-7mm in twilight. With an EPD of 5-6mm 7/8x42’s and 10x50’s will outperform 8x32’s and 10x42’s but are larger and heavier for their given magnification. 

The field of view of a binocular is dependent on the optical design and is expressed as either the width of panoramic view in metres from a distance of 1000m or in degrees where 1° is approximately 17.45m.

Wearing glasses: Many binoculars provide the full field of view when wearing glasses by either turning/pushing the retractable eyecups to the ‘down’ position or folding down the rubber eyecups. Binoculars with a stated eyerelief of 15mm or more deliver the full field of view for most spectacle wearers.


Weight For most people, small, lightweight binoculars will be used much more than large heavy ones which tend to be left at home or in the car.


Prismatic telescopes are often called spottingscopes or fieldscopes and follow the basic design of a large monocular. Opticron fieldscopes come in ‘body only’ format allowing you to choose an eyepiece or eyepieces to suit your hobby and budget.

Eyepieces are described according to their magnification when fitted to a particular spottingscope and whether they are wide-angle e.g. 20xWW or zoom e.g. 20-60x. A 20x eyepiece makes an object appear 1/20th its actual distance compared to your naked eye. Using this rule an object 500m distant appears 25m away at 20x, 16.7m at 30x and 8m at 60x.


What magnification/objective lens size? Spottingscopes are most often used for high magnification daylight observation at distance. In normal daylight when your pupil is dilated 2-3mm, a 66mm fieldscope will deliver optimum performance, (the balance between magnification and image brightness) between 22x and 35x, i.e. when the exit pupil diameter of the fieldscope equals that of your iris. In low light when your pupil dilates 5-7mm, optimum performance can only be achieved by lowering the magnification or increasing the size of the objective lens.

The higher the magnification, the greater amount of image and colour distortion. These effects can be reduced by using ED or Fluorite lenses in the objective system but run at a premium over standard optical glass lenses.

Field of view is usually expressed as the width in metres of the image when viewing at a distance of 1000m and is directly related to the magnification. The higher the magnification, the smaller the field of view. The objective lens, irrespective of diameter has no influence over the field of view.

Light transmission One way to assess the brightness of a fieldscope and eyepiece is to calculate the exit pupil diameter in the same way as with a binocular and make a trade-off between brightness and magnification desired. For general daytime terrestrial observation good compromise magnifications are 18-25x(50mm), 20-30x(60mm), 25-35x(66mm) and 25-40x(80mm).

Resolution As a general rule a good spottingscope can resolve (separate) two black dots 1.5mm distance apart on a white surface, in bright daylight at a distance of 50m.

Which Eyepiece? The vast majority of spottingscopes are used with zoom eyepieces as they offer the greatest flexibility for viewing at different distances in different light conditions. We also offer you a wide range of eyepieces for specialised applications and there is no substitute for side by side testing if you have the opportunity to do so.

Wearing Glasses Eyerelief is the distance between the eye lens and the point where your pupil is positioned to obtain full field of view and varies slightly from eyepiece to eyepiece. In some cases the eyerelief may be shorter than you need to obtain the full field of view. Almost all Opticron eyepieces offer the full field of view for most people who choose to use them with their glasses on.

Straight or 45° angled? Straight scopes are easier to use following fast moving objects, using the instrument from the confined spaces of a hide/blind or vehicle, or when hand-holding. Advantages of 45° angled spottingscopes are that your back, shoulders and neck are more relaxed when viewing and your tripod can be set lower for increased stability. Angled scopes allow people of different heights to view simultaneously without having to make any adjustment to the height of the tripod.


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