Binoculars are great for game viewing, ticking off that Hottentot Buttonquail from your birding list or checking if there is a bridge over the river down there.
But what makes one pair of binoculars better, or different, to the other and what all the numbers mean are still rather mysterious to many prospective buyers. It is necessary to understand these markings in order to be able to compare apples with apples when shopping for your next pair of binocs.
First off, binoculars use a series of lenses, elements and prisms to produce a magnified view of objects. Using two parallel optical tubes allows the user to observe with both eyes open, which is more comfortable and natural than using a scope or telescope, which requires you to keep one eye closed. Having both eyes open provides a rich and immersive experience where the scene takes on a more lifelike appearance.
When shopping for binoculars you will notice that they can vary in price quite substantially, this is often determined by the quality of the optics, the types of coatings applied to lenses and other features such as housing material or prism type.
All binoculars are described by using a pair of numbers, such as 7×50 or 8×30. The first number, including the x, represents magnification. This tells the degree to which the object observed is enlarged. For example, a 7x binocular makes an object appear seven times closer than when viewed by the naked eye.
There are certain types of binoculars that offer variable magnification, usually in the range of 5x to 8x but they do not come highly recommended due to often inferior optical quality and fragile mechanics.
Magnification is not the most important and is usually within the 7x to 12x range. If the magnification exceeds that it will not be easy to hold the binoculars steady enough. This can be remedied with a tripod mount or image-stabilized binoculars but that is not always practical.
The second number in the two number description refers to aperture. This figure represents the diameter of each of the objective lenses, which are the lenses furthest from your eye, in millimetres. Therefore 7×50 binoculars have objective lenses 50mm in diameter.
Aperture is important because like in photography, it determines the light gathering ability of your binoculars and becomes very important in low light conditions. More light means a brighter view. 35mm binoculars might work perfectly at the waterhole around lunchtime when the sun is out, but will become less effective compared to 50mm binoculars as the sun begins to set.
That 15mm might not sound like much, but one needs to compare the surface area, not the diameter. When you do the maths it becomes clear that the 7×50 binoculars will have twice the light gathering ability of a pair of 7×35 binoculars. Basically, when it comes to aperture, bigger is better.
Field of view
The field of view is the area of sky or land seen through the binoculars and is determined by the design of the instrument’s optics. It can be marked in two ways, either as width in feet at 1 000 yards or in degrees of field. When expressed in feet the field is called linear and when expressed in degrees it is called angular. Dividing the linear field by 52.35 will give you the angular field.
In most cases the field is indicated on the outside of the binoculars in degrees. Average values are between 5 and 10 degrees or approximately 260 to 520 feet.
A wide field of view is desirable for most applications however when increasing the field beyond a certain point images start to exhibit signs of distortion near the edges of the field. The field of view is also related to the magnification, the more powerful the binoculars, the narrower the field of view will be.
Exit pupil is a number that indicates how bright an object will appear when viewed in low-light conditions. A higher number means brighter images and makes it easier to maintain a full image of an object if your hands move or shake.
The exit pupil size, measured in millimetres, is calculated by dividing the diameter of the objective lenses by the magnification number. For example on 7×50 binoculars, 50 divided by 7 equals an exit pupil diameter of 7,1mm.
In very dim light our pupils can widen up to 7mm so if binoculars have an exit pupil size of less than 7 they are restricting the light available to your eyes. The 7,1mm exit pupil size of the 7×50 binoculars is thus a good choice for nighttime viewing.
For low light viewing such as dawn or dusk or within dense tree cover, models with an exit pupil number of 5mm or higher are good options. For daylight viewing exit pupil size is less important as the human pupils narrow to roughly 2mm. All binoculars offer exit pupils that size or larger.
Eye relief is the distance between each eyepiece and your eyes while the whole field of view is visible. Longer eye relief increases comfort by allowing you to hold the binoculars away from your face and is most useful for users who wear glasses. Some manufacturers recommend that users wearing glasses roll down the rubber eyepiece collars before viewing but exceptions do exist, but if you wear glasses look for eye relief of 11mm or more.
Text: Reuben van Niekerk