Understanding binoculars - terms explained

What do binocular numbers and specifications mean?

Let’s break down some of the terms you’ll come across when buying birdwatching binoculars! Plus tips of what to look out for to find the right power and features for you. 

Binocular magnification numbers

Means: How ‘zoomed in’ will your view be?
Also affects Field of view, steadiness, close focus distance, weight, depth of field (range of focus)
Options
& tips:
Usually between 7x (7 times) and 10x for birdwatching
7x or 8x = for general and garden birdwatching
10x = consider for greater distances e.g. wetlands/expansive landscapes, hawks
 

Objective lens diameter

Means: Measured in millimetres, it’s the light-gathering lens. The larger it is, the more light it can let in (so potentially a brighter image).
Also affects Size, weight, cost, field of view
Options
& tips:
Usually between 25mm and 50mm
40-50mm = better for low-light conditions (astronomy binoculars start at around 80mm, are huge and require a tripod to use!)
32mm = an effective, balanced option popular for general bird watching
25mm = light and small compact models for daytime use - test the brightness
 

Exit pupil

Means: Helps to indicate low-light performance. Measured in millimetres, it’s the Objective lens diameter divided by the Magnification.
Also affects -
Options
& tips:
Usually between 2mm and 5.25mm in birdwatching binoculars.
Low exit pupil = Don’t expect this to perform well in low light.
High exit pupil = Captures more light so potentially better in low light, but test as other factors might affect this!
 

Field of view

Means: You could describe this as how wide your view is, so a wide field of view makes following moving objects easier – particularly important when watching birds. Expensive models offer sharp images right to the edge.  Measured in degrees or a distance e.g. metres visible at a distance of 1000m.
Also affects Magnification, eye relief
Options
& tips:
Usually between 5 and 9 degrees for typical bird watching binoculars but best to try out before buying to find the right balance for you.
Lower number (say under 7 degrees)= narrower field of view. (Usually the case with higher power/magnification)
Higher number (say over 7 degrees)= wider field of view so it can be a little easier to follow and spot fast moving wildlife and birds, although you’d most likely have a lower magnification.
 

Close focus

Means: Not usually what you think of when shopping for binoculars, but important if you want to admire details of wildlife at close range - a great example is for viewing butterflies, bees and other insects close up. Usually measured in metres, close focus is the minimum distance you can be to your subject with it still being in focus.
Also affects -
Options
& tips:
Lower = better for insects, or for admiring the details on birds close to you (e.g. just outside your window if you’re at home)
Higher = Perfectly fine for more typical binocular use

 

Eye relief & eye cups / pieces

Means: The distance between your eye and the eyepiece. If your eye is too far from the eyepiece, for instance because you’re wearing glasses, you don’t see the edges. Usually measured in millimetres (and if you wear glasses, you’re looking for at least 16mm) but it’s best to test out in person before buying.
Also affects Field of view
Options
& tips:
Twist-down eye cups = found on most RSPB binoculars - mean you can adjust the eye relief to suit you whether you’re wearing glasses/spectacles or not.
Removable eyepieces = also adjustable, but if the best position is without the eyepieces on you can save the time adjusting before use.

 

Weatherproof, waterproof & nitrogen filled

Means: If you need binoculars for uses other than garden birdwatching from inside your house, these qualities may come in useful!
Also affects Cost
Options
& tips:
Weatherproof = Can use outside but avoid heavy rain situations.
Waterproof = Fully sealed, usually by replacing air with nitrogen and fully sealing. Ensure you look for waterproof if birdwatching by water!
Nitrogen filled = Prevents moisture (inc. condensation/fogging), dirt & dust particles from getting in & blurring your image.
 

Water repellent lenses

Means: Applied on the outside of the objective lenses. A water repellent coating makes your binoculars quicker and easier to clean.
Also affects Cost
Options
& tips:
Especially useful if using your binoculars in wet weather.
 

Lens coatings & glass build

Means: There are various treatments for the glass lens to improve image quality. Some are better than others so it's important to try out the binoculars in person rather than base your decision around these factors. Such treatments and coatings require expertise and quality control which is why RSPB works with Viking to produce quality optics, and also sell well-known brands such as Swarovski and Leica.
Also affects Cost
Options
& tips:
ED lens/glass (Extra low dispersion glass) = Reduces colour fringing which can increase clarity and brightness; quicker focusing.
Phase coating/correction (n/a for porro prism shapes) = Corrective contrast, brightness and sharpness/resolution.
Anti-reflection lens coatings = Fully coated, multi coated or both.
Coated = A single anti-reflection coating on some lens elements = enables more light to be transmitted to you than if there was no coating.
Fully coated = A single anti-reflection coating on all air to glass surfaces = better than a single coating.
Multi-coated = Multiple anti-reflection coatings on some lens elements = more light transmitted to you.
Fully multi-coated = Multiple anti-reflection coatings on all lens elements = ideal, more light transmitted to you.
Dielectric coating = maximises the quality of visible light and produces clear, high-contrast images, similar to those seen by the naked eye. Featured in RSPB Harrier binoculars.
 

Body build & protection (GA or RA)

Means: Some binoculars are rubber-covered (sometimes indicated by the letters GA or RA).
Also affects -
Options
& tips:
Rubber armour can make more for a more comfortable grip as well as offer some protection against knocks and wear.

 

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