There are lots of very technical issues worth knowing about binoculars. These can relate to the user’s visual status, physical ability, conditions of use, purpose of use etc. Some of these will be explained in this article. However, if I was only allowed to give one piece of advice in general terms, it would be to buy an 8 X 40 model. Higher magnification does not necessarily make for better viewing, because the higher the magnification, the more the residual movement (shake) is magnified when holding the binoculars, and this will corrupt the clarity of the object viewed. In other words, an 8 X magnification will give a sharper image than a 10 X, unless the 10 X is placed on a tripod to eliminate shake. Moreover, as magnification increases, brightness and clarity may diminish, the depth of field may be shallower and the field of view is usually more restricted. Distortion by heat waves and other effects of atmospheric conditions are also more noticeable at higher powers.

Another point worth mentioning upfront, is when buying binoculars, you get what you pay for. Buying the cheapest is not the smart option. However, top-end binoculars can be frightfully expensive (towards R30 000), and one can do very well between R4000 – R7000. A lot can be said about the design of binoculars as well as the quality of coatings on the lenses. This is essentially where the money is spent. The more you pay, the better the performance one can expect.

 Let’s demystify some of the technical issues of binoculars.

What do the numbers mean?

7 X 35 – 8 X 40 – 10 X 40.
The first number indicates the magnification. The second number denotes the diameter, in millimeters, of the objective lens.

The objective lens

The size of the objective lens (the larger end of the binocular) is almost always directly related to the size of the binocular. A larger objective lens will gather more light and, theoretically, allow you to see greater detail and a brighter image. A small difference in aperture will have a greater impact on the light gathering ability of a binocular than you might expect, especially under low light conditions. The apertures of compact models generally range from 20mm to 25mm while “giant binoculars”, used mainly for astronomy, may range from 70mm to 80mm in diameter.

The exit pupil

This is calculated by dividing the objective lens number by the magnification number. For example, an 8 X 40, will have an exit pupil of 5mm.

You can actually see the exit pupil as a circular image of light in the eyepiece, when you hold the binocular at arm’s length. These exit pupil images should be exactly circular, if not, I would be less confident about the overall quality of the binoculars. When the exit pupil of the binocular is larger than the eye pupil, some of the light coming from the binocular will fall on the iris and go undetected by the observer. When the exit pupil is smaller than the eye pupil, the amount of light falling on the retina will be less than that collected with your normal vision.
The image will appear dim and since resolution and contrast are adversely affected, the image appears less clear. Knowing the exit pupil, can help you choose a binocular that is well-suited to your activities. You may want to use a binocular with a large exit pupil (7 X 50), for activities like boating, during which it is hard to keep a steady hand. It is much easier to keep the smaller pupil of the eye centered in the larger exit pupil of a binocular.

What is the field of view?

When you look through your binoculars, the widest dimension you can see across the circular view is known as the field of view. The field of view is primarily determined by the design of the eyepiece and not the size of the objective lens. As a general rule, the field of view will decrease as magnification increases. For example, a 10 X model will have a smaller field of view than a 7 X model of the same design. The field of view is typically measured at 1000 yards (304,8 m) from the point of viewing. You may find the measurement listed on the binos in either feet or degrees.

Eye Relief

Binoculars are essentially designed to be used without spectacles or sunglasses, when the eyes can get close-up to the eye pieces. Wearing spectacles or sunglasses, the eyes will of course be positioned further away from the eye pieces and reduce the field of view. This is not a nice situation, but most binocular designs overcome this dilemma with adjustable eye pods. The eye pods can dial in when wearing specs, or out when not.

Twilight Factor

During daylight hours the magnification of a binocular will be the principal factor in image resolution. At night, when the eye pupil is dilated, aperture size is the controlling factor. In twilight conditions, both of these factors affect resolution. The twilight factor compares binocular performance under these conditions. A higher twilight factor indicates that the binocular will resolve images better under dim light conditions.

Calculate the twilight factor of a binocular this way:
1) Multiply the magnification by the aperture
2) Find the square root of this product

Calculation of the Twilight Factor

According to this indicator, a 10 X 40 model (twilight factor of 20) would resolve better than a 7 X 35 model (twilight factor of 15.4) even though the 10 X 40 has a smaller exit pupil. Note that the twilight factor will primarily indicate performance at dawn or dusk without consideration of the light transmittance or glass quality of the binocular.

Trouble shooting and care

Make the effort to understand how to focus your binoculars properly (refer page 33). In my experience, most people don’t. It would be pointless to spend good money and then not get the best performance out of your purchase. Even top-end binos will perform poorly unless
focused properly.

The distance between our eyes (pupils) differs from person to person. It is very important to get the eye pieces to align exactly with our pupils. This can be done by pushing the two sides of the binos together or pulling them apart, while looking through the binos. You can immediately tell, by the quality of the image when alignment is correct.

Keep the objective and eye piece lenses clean. Many older people don’t see well at near and often they can’t determine if the bino lenses are clean. Dirty lenses can be very problematic. Do not clean with a dry cloth or tissue paper. Use a micro cleaning cloth with lens spray cleaner.

Beware of beach sand in your bino pouch. This will transfer onto the bino lenses and can cause scratches.

Always ensure that the eye pods are equally and fully dialed in or out. If you are battling to find a well-focused image, this is the first thing to check. If there is an imbalance of where they are positioned, it will cause a blur.

Visual correction

Good vision with your binos will be a function of the refractive status of your eyes. It makes sense to ensure that you start from the right basis by having a comprehensive visual assessment by an optometrist. The optometrist will have clever options for the keen birder or hunter, such as contact lenses, just for those particular activities.


Purchasing a pair of binoculars can be a life time investment. It is best to spend the money to get the quality. Unfortunately, in my experience, binocular sales people, seldom have the knowledge to advise properly on all the factors that should be taken into account. Someone needs to know the facts to ensure that your purchase will best suit the activity you want to indulge in. To this end, it may be well worth the trouble to familiarise yourself with the key issues.

Focusing your binoculars

To focus your binoculars properly, follow these easy steps:

  1. Turn your main focus knob all the way to the right.
  2. Locate your diopter focusing piece. This is either another focus knob or a movable eyepiece, usually the right eyepiece.
  3. Choose a distant stationery object on which to focus.
  4. Looking through the binoculars, close your right eye (or cover the right lens) and, using the center focus wheel, adjust the focus until a clear image appears. You may need to adjust both wheels slightly to achieve the sharpest degree of focus.
  5. Close your left eye (or cover the left objective lens with your hand) and quickly turn the diopter focus piece, so that the image is clear and in sharp focus for your right eye. Lower the binoculars and rest your eyes for a moment.
  6. Once you’ve achieved maximum focus, look at the settings for your diopter adjustment (most binos have symbols or number to indicate settings).
  7. Lastly, ensure that the eye pieces are aligned with your inter-pupillary distance, by pulling or pushing on the sides of the binos to move the eye pieces in or out. You will be able to tell immediately when this setting is right for your eyes.

Remember where your optimal setting is, so you can automatically readjust your binoculars to that point should the setting be changed by another user.

Once you get your binos focused and the dioptre adjusted for your own eyes, the only focusing you have to do is with the centre focus wheel. For most birds that appear in the middle distance, say 30 to 60 feet away, you don’t need to refocus at all. Closer or more distant birds require some focus adjustment with the center wheel, however.

Properly focused, your binoculars will give you a crystal-clear image, and that’s what bird watching is all about, after all.

Share this on Facebook: