Binocular Terminology Guide: Understanding Common Binocular Terms
If you've been researching binoculars, you've probably come across technical terms such as 8x42, ED glass, field of view, and exit pupil. For newcomers to birdwatching and wildlife observation, this terminology can be confusing.
The good news is that understanding a few key binocular terms can make choosing the right binoculars much easier.
In this guide, we'll explain the most common binocular terminology in simple language, helping you make a more informed purchasing decision.
What Do the Numbers Mean? (8x42, 10x42, 8x32)
One of the first things you'll notice when looking at binoculars is a specification such as:
- 8x42
- 10x42
- 8x32
- 10x50
These numbers describe the binocular's magnification and objective lens size.
Magnification
The first number indicates magnification.
For example:
- 8x means the image appears eight times closer.
- 10x means the image appears ten times closer.
Higher magnification can reveal more detail but may also make the image less stable when hand-held.
Objective Lens Diameter
The second number refers to the diameter of the front lens in millimetres.
For example:
- 42 means a 42mm objective lens.
- 50 means a 50mm objective lens.
Larger objective lenses generally allow more light into the binocular, producing brighter images, particularly in low-light conditions.
Field of View (FOV)
Field of view describes how much of the landscape you can see through the binoculars.
A wider field of view helps you:
- Locate birds more quickly
- Track moving wildlife
- Observe larger areas
Birdwatchers often prefer binoculars with wider fields of view because birds rarely stay still for long.
Exit Pupil
Exit pupil refers to the small circle of light visible when you hold binoculars at arm's length and look through the eyepieces.
The larger the exit pupil:
- The brighter the image appears
- The easier the binoculars are to use in low light
Example
An 8x42 binocular has:
42 ÷ 8 = 5.25mm exit pupil
A 10x42 binocular has:
42 ÷ 10 = 4.2mm exit pupil
This is one reason why many birdwatchers prefer 8x42 binoculars for dawn and dusk observation.
Eye Relief
Eye relief is the distance your eyes can be positioned from the eyepiece while still seeing the full image.
This is particularly important for people who wear glasses.
Long eye relief generally provides:
- Greater comfort
- Easier viewing
- Better compatibility with spectacles
Most quality birdwatching binoculars feature adjustable eyecups to help achieve the correct viewing position.
Close Focus Distance
Close focus distance refers to the nearest point at which the binocular can focus clearly.
A shorter close focus distance is useful for observing:
- Butterflies
- Dragonflies
- Insects
- Garden wildlife
Some binoculars can focus on subjects only a few metres away.
ED Glass (Extra-Low Dispersion Glass)
ED glass is one of the most desirable features found in modern binoculars.
ED glass helps reduce colour fringing and improves image sharpness.
Benefits include:
- Improved clarity
- Better contrast
- More accurate colours
- Enhanced detail
This is particularly useful when observing birds against bright skies or high-contrast backgrounds.
Lens Coatings
Binocular lenses are coated to improve light transmission and image quality.
Coated
Basic anti-reflective coating applied to at least one lens surface.
Fully Coated
All air-to-glass surfaces have a coating.
Multi-Coated
Multiple coatings applied to selected surfaces.
Fully Multi-Coated
Multiple coatings applied to all air-to-glass surfaces.
Fully multi-coated optics generally provide the brightest and sharpest images.
Prism Types
Most binoculars use one of two prism systems.
Roof Prism
Roof prism binoculars are:
- Compact
- Lightweight
- Modern in appearance
Most birdwatching binoculars today use roof prism designs.
Porro Prism
Porro prism binoculars feature a wider body shape and often provide excellent depth perception.
Although less common today, they still offer excellent optical performance.
Phase-Corrected Prisms
In roof prism binoculars, phase correction coatings help improve image sharpness and contrast.
Benefits include:
- Better detail
- Improved colour fidelity
- Enhanced image quality
Most mid-range and premium binoculars feature phase-corrected prisms.
Dielectric Prism Coatings
Dielectric coatings improve light reflection within the prism system.
This results in:
- Brighter images
- Improved contrast
- Enhanced overall performance
Dielectric coatings are often found on premium binocular models.
Waterproof Binoculars
Waterproof binoculars are sealed to prevent moisture entering the optical system.
Advantages include:
- Protection from rain
- Increased durability
- Better long-term reliability
For birdwatching in the UK, waterproof binoculars are highly recommended.
Fog Proof Binoculars
Fog proof binoculars are filled with nitrogen or argon gas.
This prevents:
- Internal condensation
- Lens fogging during temperature changes
A fog-proof design is particularly valuable during cold mornings and damp conditions.
Nitrogen Purged
Nitrogen purging removes moisture from inside the binocular before sealing.
Benefits include:
- Preventing internal fogging
- Improved waterproofing
- Increased longevity
Many premium binoculars use nitrogen purging as standard.
Interpupillary Distance (IPD)
Interpupillary distance refers to the distance between your pupils.
Binoculars can be adjusted to match your eye spacing, ensuring:
- Comfortable viewing
- Full image visibility
- Reduced eye strain
Dioptre Adjustment
The dioptre adjustment allows users to compensate for differences in vision between their left and right eyes.
Once adjusted correctly, images appear sharper and more comfortable to view.
Most binoculars feature a dioptre ring near one eyepiece.
Chromatic Aberration
Chromatic aberration appears as coloured fringes around objects, particularly in high-contrast situations.
ED glass and premium optical systems help reduce this effect significantly.
Brightness
Brightness refers to how much light reaches your eyes through the binocular.
Factors affecting brightness include:
- Objective lens size
- Lens coatings
- Prism quality
- Exit pupil size
Brighter binoculars perform better during dawn, dusk, and poor weather conditions.
Image Sharpness
Sharpness describes how clearly fine details can be resolved.
Factors affecting sharpness include:
- Optical quality
- Lens coatings
- ED glass
- Prism coatings
High-quality optics allow birdwatchers to identify species more easily and enjoy a more immersive viewing experience.
Which Specifications Matter Most for Birdwatching?
For most birdwatchers, the most important specifications are:
- 8x or 10x magnification
- 42mm objective lenses
- ED glass
- Fully multi-coated optics
- Waterproof construction
- Phase-corrected prisms
- Comfortable eye relief
These features combine to provide excellent performance across a wide range of habitats and lighting conditions.
Final Thoughts
Binocular specifications may seem overwhelming at first, but understanding a few key terms can make choosing the right binoculars much easier.
For most birdwatchers and wildlife enthusiasts, an 8x42 binocular with ED glass, fully multi-coated optics, and waterproof construction offers an excellent balance of performance and value.
At Nature View Optics, we stock a carefully selected range of binoculars from leading brands including Hawke, Vortex, and other trusted manufacturers. If you need help choosing the right binoculars, our team is always happy to provide expert advice.