Compact binoculars: Features explained
Binocular types
Porro prism binoculars have an ‘M’ shape design
There are two main types of binocular design, porro prism and roof prism. Each differs in the way the prisms channel light through the binoculars to your eyes. Porro prism binoculars are characterised by a ‘M’ shape design, while roof prism designs are ‘H’ shaped.
Although the Best Buy binoculars in our test are all of porro prism design, there is not thought to be any difference in performance based solely on the binocular type.
Optical features
Magnification
The model name of binoculars, (often written on the chassis of the model), is followed by two numbers that indicate their specification. The first number is the binoculars’ magnification. This tells you how many times larger an image will appear, compared with that seen by the naked eye.
A greater magnification doesn’t mean image quality is improved, only that the size of the image appears larger.
Objective lens diameter
The second number after the model name refers to the objective lens diameter. This is a measurement of the lenses at the end of the binoculars – the place where light enters, recorded in millimetres.
The larger the measurement, the more light the binoculars can take in. This will make the image appear brighter, especially in dim light conditions, (all other factors being equal).
Field of view
The field of view is the width of the area you see though the binoculars at a distance of 1000 meters, and is usually measured in degrees or meters.
Generally, the higher the magnification the smaller the field of view. However, specially manufactured ‘wide angle’ binoculars can give a field of view greater than binoculars with a comparable magnification. The advantage of a larger field of view is that it’s easier to spot and follow things, for example a bird in flight, or your horse and jockey approaching the finishing post.
Exit pupil
Compact binoculars perform best in bright, daylight conditions
The exit pupil measurement tells you how much light is sent to your eye through the binoculars, and can be found by dividing the size of the objective lens by the magnification. The larger the exit pupil, the brighter the image.
As the binoculars in our test are compact models, the exit pupil measurements are fairly small. This means they will perform best in bright, daylight conditions.
Close-up focus
The shortest distance to achieve an image in focus. The shorter the distance, the better for focusing on small wildlife.
Light transmission
The amount of light that reaches the eye after entering the binoculars. Lens coatings can affect the light transmission in binoculars (see Coated optics).
Eye relief
The maximum distance your eye can be from the eyepiece and still see the whole field of view. Binoculars which are intended to be used by glasses wearers will have a long eye relief. This will allow them to see the whole field of view.
Twilight factor
A mathematical calculation used to predict the performance of binoculars in low light conditions.
Collimation
How parallel the two binocular tubes are. The more perfect, the less the eye strain or possibility of a double image.
Physical features
Dioptre adjustment
Most binoculars have a central wheel and a dioptre adjustment for focussing images. Usually located on the right eyepiece, the dioptre adjustment is an important feature used to compensate for differences between the strength of the eyes. However, it can often go unnoticed by novice users who are unsure of its use and only adjust the central focussing wheel.
Read our quick setup guide in the FAQ section to see how to focus your binoculars.
Eyecups
Eyecups are located at the entrance of the binoculars barrels (where you look in). When placed in the ‘up’ position they help to block out lateral light when using the binoculars. Glasses users can fold these ‘down’ to allow them to see the whole field of view.
Added extras
Armour covering
This refers to binoculars that have their chassis covered by a synthetic material, usually rubber. This is intended to improve comfort and prevent scratches.
Coated optics
The image quality of binoculars can largely depend on the lenses used. Lens coatings can help to reduce the amount of reflected light and increase the amount that reaches the eye (light transmission) – improving the brightness and clarity of the image.