Binoculars Glossary

Buying a pair of binoculars for bird watching can be daunting, not least because of all the terms and abbreviations that are used to describe them.

People Using Binoculars

Here is a glossary of some of the most common terms which you may find useful when deciding which binoculars to buy.

Anti-Reflective Coating
A chemical coating applied to the surfaces of selected lenses and prisms to reduce unwanted reflections. This improves light transmission, giving a sharp, bright, high contrast image whilst reducing flare.

Aspherical Lens (AL)
A lens element that drastically reduces the spherical aberration often found in large-diameter, wide-angle lenses. It also minimises chromatic aberration and distortion, thus improving image quality.

Bak4 Glass Prism
A binocular prism design that produces a perfectly round exit pupil and ensures greater light-gathering power for clear, images without any vignetting (where the image’s brightness is reduced at the periphery compared to the centre).

Dach Centre Focus, which describes the roof-like shape of the internal prism of roof-prism binoculars; Dach being the German word for roof.

Dioptre Adjustment
Allows glasses wearers to use binoculars without their spectacles.

Doublet Field Flattener
A double lens group that compensates for curvature of field, resulting in excellent image flatness and clarity.

One or more lenses has been treated with multiple layers of a special coating that result in anything from improved light gathering to a scratch resistant surface.

Inner Focusing Mechanism (IF)
Moves only certain lens elements during focusing to minimise lens extension.

Long Eye Relief
Allows users who wear glasses to see the entire field of view, where the eye is further away from the eyepiece.

Mini Centre Focus, whereby the porro prism design is altered to produce objective lenses that are closer together than the eyepieces for a more compact binocular.

Optical Image Stabilizer
A mechanism that helps to increase the stability of an image; particularly useful when using high powered binoculars or when viewing from a moving object such as a boat or car.

Porro Prism
A combination of right-angle prisms that ensures the image is the right way up for viewing. Porro prism binoculars have the objective lens and the eyepiece lens off-set, combining higher magnification and brighter imaging with cost-effectiveness. Since the objective lenses are further apart, three dimensional imaging is enhanced.

Roof Prism
Roof-prism binoculars have the objective lens and the eyepiece lens in a straight line (as opposed to a Porro prsim, in which they are offset). This gives the barrel a streamlined appearance and results in smaller, lighter designs.

Rubber Coating
Makes the binoculars easier to grip and cushions them against shocks.

Super Spectra Multicoating
Extremely thin layers of chemicals applied to the surfaces of selected lenses and prisms to reduce unwanted reflections. This improves image sharpness, contrast and colour balance.

Twilight Factor
A measurement of viewing efficiency and image detail in low lighting conditions.

Will survive submersion up to one metre underwater for 5 minutes.

Weather Resistant
Splash proof; cannot be immersed in water. (JIS Grade 4) Higher JID grades (5 and 6) can be used in rain and can be washed in water (e.g. under a tap).

Binoculars with an apparent field of view greater than 65º offering a broader scope of view.

An abbreviation for Zeiss Centre Focus, which employs the porro prism design, where the objective lenses are furher apart the eyepieces.

Zoom Lens
A lens that allows the focal length to be altered. A magnification of factor 8, for example, produces an image as if one were 8 times closer to the object. Larger magnification leads to a smaller field of view.

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