Central Focus

Binoculars with central focusing include a mechanism that allows both eyepieces to be focused at the same time which enables faster focusing, extremely useful when you are following fast moving objects. Central focusing is enabled through the use of a wheel that is found in the middle of the binoculars. Quality of engineering has a significant impact on  ease and speed of focus. (See also – Diopter Adjuster)

Close / Near Focus

Close focus distance indicates the distance from which you can focus on an object. The importance of close focus is dependant on what you want to use your binoculars for. Obviously the close focus distance is far more important for nature watching than it is for long distance viewing such as astronomy.

Coated Lenses

Coated lenses are lenses that are coated in a thin layer of anti reflective coating in order to help reduce reflections and improve on light transmission. Most binoculars have lenses which are coated in one form or another with the greater the number of coatings and the more surfaces that are coated the higher the quality of binocular. The best binoculars have fully multi-coated lenses which means all air to glass surfaces are coated in multiple layers of anti-reflective coating  … more on lens coatings

Contrast

When we talk about contrast we are referring to the binoculars ability to differentiate between dark and brighter images and between objects and their background. When you are trying to view distant objects with your binoculars, and you want to see the finer detail of the image, the level of contrast your binoculars provides is important. Contrast is impacted by (among other influencing factors):

·        The quality of the lenses

·        Resolution

·        The quality of the eyepiece

·        The quality of the prisms

Depth of Field

The depth of field indicates the element of the image which appears sharp to the eye. Although binoculars can only precisely focus on one distance the greater the depth of field the more of the total viewing area appears sharp to the eye.

Digital Binoculars

Digital binoculars are binoculars that come with a built in camera. If you buy binoculars and camera combined each element doesn’t tend to be of the same quality as if you purchased them on their own (i.e. just binoculars or just a digital camera). By buying them as a combined package there is a compromise to be made.

Diopter Adjuster

All good quality binoculars provide you with the ability to separately adjust the focus on one eyepiece (generally the right eyepiece), in addition to central focusing, through the use of a diopter adjuster. This facility enables you to compensate for eyesight differences between the two eyes.

Exit Pupil

The Exit Pupil Diameter is always quoted, in millimetres, in the binocular specification chart. The measurement represents the beam of light, in diameter, which leaves the eyepiece. A large exit pupil will mean the images will appear brighter so for example binoculars with an exit pupil of 4.2mm should theoretically deliver brighter images than binoculars with an ext pupil of 2.8mm.

If the exit pupil isn’t quoted, it’s easy to calculate by taking the objective diameter (say 42 if the binoculars are 10x42’s) and dividing it by the magnification power of the binoculars (10 in this example). The exit pupil of 10x42 binoculars is therefore 4.2mm.

Having a large exit pupil is useful in poor light conditions so astronomy binoculars are likely to have a larger exit pupil that binoculars designed for good light conditions.

The exit pupil should equate to the dilation of the pupil in your eye after your eyes have adjusted to the light conditions (greater dilation in poor light).

Eye Relief

Eye Relief is the distance in millimetres, from the eye, that the binoculars can be held with the complete field of view still comfortably visible. When you are looking to buy a pair of binoculars you will find that some are suitable for spectacle wearers while others are not. Obviously the further away from your eye that you can hold the binoculars the more comfortable it is, particularly for people who wear glasses. Most manufacturers only recommend binoculars with an eye relief greater than 15mm for spectacle wearers.

Eyecups

Eyecups are used to ensure the correct positioning of your eyes, enable optimal eye relief and to improve image quality by reducing the amount of unwanted light. Binoculars come with rubber eyecups (that can be folded down when used with glasses) or with twist up eyecups which enable multiple positions. You can also get manufacturer specific variations including interchangeable eyecups enabling the binoculars to accommodate varying viewing conditions as well as spectacle wearers.

 

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