As a skilled outdoors expert, whether you are a hunter, sailor, or bird watcher or have used binoculars professionally for many years, you probably know exactly how to tell a good pair of binoculars from a bad one.
But it is much harder for beginners to find their way around binoculars and to understand the various features, terms, and their importance in what makes a good set of binoculars.
Often magnification is promoted as the most important criterion. But there are more essential features. The optical performance depends on high-quality glass, clever design, and precise workmanship.
Only when the properties of the glass, the coatings, and the mechanics harmonize with each other, will you get a good pair of binoculars, which produce high-resolution and sharp images without color distortion.
The nice thing about binoculars and their optical performance is that it entirely depends on physics and the laws of optics.
What to look for in a good pair of binoculars?
Optical instruments are all about good glass and coatings, it’s what makes them work. The optical glass and its composition that make up the prisms and lenses of an optical instrument are the major factors that influence the brilliance and clarity of the images.
On its way through binoculars, light has to pass through a fair bit of glass. The light that enters a lens needs to find its way into and through the glass molecular structure. On its way traveling through it, the light changes. This causes errors (aberration) in the projected images.
More precisely, Refraction, Absorption, and Transmission alter the light depending on the wavelength (colors) and the type of glass through which the light moves.
Designers of binoculars use various optical glasses with specific properties and combine those to virtually eliminate these errors.
The best glass types are Extra-Low-Dispersion Glass (ED), that’s a Calcium-Flouride Composite Crown Glass
Just as important as using high-quality optical glass are the coatings of the glass surfaces. This reduces the loss of light when the light passes from one medium to another (air into glass).
Glass may seem translucent to our eyes, but it is not for light. While most light is transmitted, some will be reflected. Normal glass reflects about 5% of the incident light. Considering the number of optical components in binoculars, the loss of light is alarming and without measures, only about 50% of the incident light would exit the binoculars again to reach the observer’s eye. Viewed images would be very dark.
To minimize this effect, thin coatings are applied to the glass. Optical glass coatings consist of thin layers of metal oxides deposited on the surfaces of lenses and prisms. These materials have a low refractive index and thus reduce the light reflection on the glass surfaces and increase the light transmission.
Coatings range from a simple one-layer anti-reflective coating to a fully multi-coated, and phase-correcting coating. The best high-quality coatings can increase the transmission to 95% or more, providing image brightness and thus also the contrast and detail richness of the image.
There are other coatings like hardening the exterior lens for scratch resistance and lotus effect to let water droplets roll off the glass surface
The binoculars collecting lens (front lens) produces an upside-down and reversed intermediate image. The prisms have the task to correct the image again. They also allow the binoculars to be shortened due to the internal back-and-forth reflection of the light beam path.
Here, too, the brightness of the projected images is reduced through reflection and transmission losses. This can be reduced by applying high-quality coatings to the surfaces of the prisms and by using quality optical glass.
The most widely used are the crown glasses BK7 and BaK4.
BK7 is a standard optical borosilicate glass, commonly used for lenses and prisms. Bak4 is a Barium crown glass with better properties, it has a higher transmission and provides better resolution and color-true images. Since BaK4 glass is more complex to manufacture than BK7 glass, this is often only offered in higher-priced models.
The magnification of binoculars indicates how much an object appears closer and larger. With a 10×42 glass, the object is magnified 10-fold, so if an object is 100 yards away, it appears only 10 yards away.
The smaller the magnification the larger the field of view. The larger the magnification the smaller the field of view.
The models with 8x or 10x magnification are the most popular. Smaller magnification than 8x makes sense if one needs to observe a large area as the lower magnification offers the advantage of a larger field of view. A seafarer might scan for obstructions or a hunter can search out moving prey.
Binoculars with a larger magnification of 12x or more have a smaller field of view and allow seeing objects in more detail. Useful for birdwatchers or anyone who needs to get a detailed view of small distant objects. The disadvantage of large magnifications is to keep the binoculars steady, a tripod may become handy or buying a pair of binoculars with image stabilization.
The front lens of the binoculars is called the objective lens. The second value of the binocular description e.g. 8 x 42 means that the front lens has a diameter of 42 mm.
The larger the lens diameter (aperture), the more light can be collected. The projected image can be of higher resolution, be richer in contrast, and will appear brighter.
Handheld binos range in aperture from about 20mm to just over 60mm. Anything wider becomes too heavy. Compact binoculars have lens diameters of about 20mm to 25mm, this is sufficient for use in good daylight.
An objective diameter of 30-42 mm is the most popular in binoculars. It enables excellent observation during the day and offers sufficient light gathering even in poor light conditions, such as twilight. 50mm and more are best for use in very poor light conditions at dusk or at night.
The brilliance and brightness of the projected images depend not only on the lens diameter but also very much on the type of glass used and the coating of the lenses and the prisms. Prime glass and lenses are high-tech products and greatly influence the price of binoculars.
The diameter of the light beam that exits the eyepiece and hits the eye of the observer is the exit pupil.
The calculation of the exit pupil is quite simple. To do this, simply divide the lens diameter by the magnification, the result is the diameter of the exit pupil in millimeters.
The exit pupil in optical devices is usually between 2 and 7 mm. Larger makes no sense because the pupil of the eye does not open more than 7 millimeters and even that far only in a young person’s eye.
With age, the ability to dilate diminishes. Smaller than 2 mm makes no sense either as the projected images become too dark then.
The average diameter of the exit pupil of most binoculars is about 4 millimeters. Larger exit pupils are found in models that are designed for low light conditions, but even this only benefits people with young agile pupils.
To be able to see the full field of view, the eye of the observer must not be too far away from the binoculars. Otherwise, the projected image can not be fully seen. The maximum distance up to where the image can be fully seen is called eye relief.
Typically that distance is in most binoculars around 15mm, give or take a bit.
For binoculars to be used while wearing eyeglasses, enough eye relief is required to accommodate the extra distance the glasses add. This ensures the projected image is not being displayed in front of the eyes and can then only be partially viewed. Ideally is an eye relief of 16mm or more.
Field of view
The field of view is the width of the image shown in the binoculars. It is usually expressed in degrees or in the width in feet (i.e. yards, meters) per 1,000 yards (or 1,000 meters) of the horizontal axis through the image.
If the field of view is 120-yard then you see in 1000 yards distance only a 120-yard wide area.
The stronger the magnification, the smaller the field of view, it is easier to observe objects in detail. Lower magnification results in a larger field of view, making it easier to survey a large area or track a moving object.
A wide field of view has also disadvantages. Spherical aberration and distortion near the edge of the images become more apparent. This can be compensated but the more complex optical construction, better coatings, and glass types will add to the cost.
The idea of binoculars is to observe distant objects. Due to the design, it is not possible that binoculars can focus from 0 to infinity distance. There is a minimum focusing distance, the Close Focus. This is the minimum distance an object must be away from the binoculars so it is still possible to focus on it. This can be as low as a couple of feet.
Most people might rarely never need close focus. However, it is an interesting feature for birdwatchers, nature observers, or scientists who also like to observe birds, butterflies, insects, and small creatures nearby.
The close focus can not be easily calculated as it depends on many factors. One must refer to the specs and information provided by the manufacturer.
The twilight number should give an indication of the performance of the binoculars in low-light conditions. It is the square root of the product of magnification and lens diameter. TF=√M Do
Depending on the binocular specification, the value is somewhere between 5 and 25. The higher the value, the better the performance in low light conditions and the brighter the projected images.
The twilight number has always been an important criterion in how to tell a good pair of binoculars. However, today’s high-tech materials have added new variables that greatly affect the performance in low light. Technical details such as the type of optical glass, the anti-reflective coatings, and the type of prisms, all have a high influence on the transmission of light and thus on the performance of the binoculars.
Waterproof and anti-fog properties are great features in any binoculars. It protects the device from harsh environments like in the mountains, at sea, in the jungle … almost everywhere. In addition to preventing water from entering, the anti-fogging feature prevents internal condensation of moisture due to temperature fluctuations. A problem that may arise with unsealed binoculars when they are brought into the house from the cold outside.
O-ring sealing and gas purging of the interior of binoculars whit nitrogen or argon is a standard in most premium models.
Depending on where and how you use the binoculars, weight can be more or less important. If you hike and only need the instrument for occasional observations, lightweight compact binoculars are a better choice.
Although lightweight binoculars have the advantage that you can hold them up for a long time without getting tired, they have the disadvantage that they are more prone to tremble through the lower mass inertia.
In this regard, a heavier model is better because it is more stable in your hands and allows tremble-free viewing. The downside is that you tire pretty quickly. From a weight of 3 to 4 pounds, a tripod is recommended for longer observation.
The small compact binoculars weigh only about 7 to 10 ounces. The usual standard binoculars weigh about 2 pounds give or take, which is a good compromise for handiness and stability. Larger models for use in low light or astronomy may exceed 4 pounds.
So what is a good pair of binoculars?
We have seen that the performance of an optical device depends on many factors and properties. Manufacturers of really good binoculars have to take all of this into account.
Buying an instrument that combines high-quality materials with the best workmanship is not cheap. But binoculars that cost less than $ 1,000 can do well too. Provided that you pay attention to the essential features.
- The quality of the optical glass and its coatings must be considered first if your priority is high transmission, color fidelity, image contrast, and sharpness.
- Then look at the magnification, the aperture of the objective, the field of view, eye relief, and the mechanics.