How do binoculars work? All is revealed.

Perhaps you use binoculars regularly during your hunting expeditions or when observing animals. But do you know specifically what parts make up a pair of binoculars and how light is transmitted inside them? We will explain it all to you here.

How do binoculars work?

The light transmission percentage is a fundamental criteria of a good pair of binoculars. But how does this transmission work? What treatments do we apply to our 100, 500 and 900 models? What are the important criteria to know? The answers are in the article.

The basic concepts of a pair of binoculars

To properly understand our design choices, it is necessary to focus on how binoculars work. To this end, we are first going to explain two fundamental optical principles: the first concerns the correlation that can exist between the type of binoculars and their brightness.

A pair of binoculars has two numbers. For example, 10x42. The first number, 10, is the magnification. The second, 42, is the diameter of the objective lens. If we take the ratio of the two, we get a value in millimetres. So, for example, 42/10 gives 4.2mm. This is what we call the exit pupil. In concrete terms, the exit pupil corresponds to the beam of light that comes out of the binoculars and touches your eye.
If, then, we take a 10x25 pair of binoculars, the exit pupil is 2.5mm. In an 8x56 pair of binoculars, the exit pupil is 7mm. This information is vital, and you’ll understand why.

How do binoculars work?

The eye, a complex organ

Let’s focus on how the human eye works. The eye is made up of rods and cones. Rods are photoreceptors which convert light into nerve signals: in white, black or grey. Cones are responsible for colour. If the eye receives only a little light, we will have grey-scale vision. If, on the other hand, the eye receives a lot of light, we will perceive all the colour nuances possible to see. So, the function of the exit pupil is to ensure that the beam of light leaving the binoculars fully lights up the pupil of your eye.

How do binoculars work?

However, where this gets complicated is that the eye pupil contracts or dilates depending on the ambient brightness. So, in the middle of the day, the pupil is around 2-3mm in diameter, in the dark it is 7-8mm in diameter. And this varies depending on age: when you are 18, the dilated pupil is 8mm in diameter, between 18 and 30, it is 6-7mm in diameter, between 30 and 50, it is 5-6mm, and at 60/70 years old, the pupil is around 5mm in diameter.

Another important point: we often hear from parents who take their children along for wildlife observation say that they don’t have the same vision through the binoculars. This is linked to the fact that children have a pupil that dilates more. This means that a 50 year-old adult might be able to see vividly using binoculars, while their child might instead see in grey scale / without colour. As a result, depending on age and the observation conditions, each type of binoculars has its uses.

Circulation of light in your binoculars

The second optical principal interesting to tackle is the circulation of light in a pair of binoculars.
On the X-ray image of this pair of binoculars, you can see the objective lens at the bottom. Light passes through the objective lens, then the focus lens (which makes the image sharply defined), then the prism, and then the eyepiece lenses. What you must remember is that light is transmitted in a straight line. As such, there are obstacles in a pair of binoculars. For example, the optical components. So, on contact with an obstacle, the light can be absorbed, dispersed, reflected, or refracted. The light is going to encounter six, seven or eight optical obstacles and what you should know is that, at each of these interfaces, the light loses around 5% of its transmission rate. As it stands, if no action is taken, 60% of the light which enters the objective lens would leave the eyepiece. You would then have very little contrast, and it would appear quite dull and rather dark when you looked through the binoculars. The main aim in designing a pair of binoculars is therefore to improve the definition and quality of the image.

How do binoculars work?

How to improve the image quality of a pair of binoculars?

There are several ways to do this. The first is to make adjustments to the lenses. We apply treatments to the lenses, particularly magnesium fluoride, which reduces the diffraction of the light. For our new binoculars 100, we have an MC (Multi-Coated) treatment which allows us to reach a light transmission rate of close to 70%. Looking at our new binoculars 500 models, we have applied an FMC (Full Multi-Coated) treatment to all the lenses, which gives a light transmission rate of 78%. In the 900 range, we have an FBMC (Full-Broadband Multi-Coated) treatment for a light transition rate of up to 83%.

How do binoculars work?

So, maybe you’re wondering: why don’t we use the best treatments from the entry level, from the 100 range, for example? Note that the treatment is the most expensive part of a pair of binoculars. Despite what you may think, it is not the frame or even the grip which cost the most ... it is actually the mineral lens and its treatment. The cost of the treatment for an FMBC pair of binoculars is 15 times higher than that of an MC pair of binoculars (the difference therefore between our 900 and 100 level models). Yes, this has a direct impact on the sale price, but also on the performance of the binoculars. In order to maintain accessible prices while still providing high-quality products, we offer models with different treatments. This is where the range of levels 100, 500 and 900 comes from.

And the transmission rate of different prisms alongside all that?

In the diagram below, you can see four curves: the curves show how the rate of light transmission through a prism with a treatment layer varies according to the wavelength of the visible spectrum (the visible spectrum ranges from 400 to 700 nanometres). We have applied an aluminium-based treatment to the prism of our new 100 binoculars. This is the green curve on the diagram. The diagram shows that the transmission rate is optimal with the colours blue and green; on the other hand, it is less marked for warm colours, meaning orange and red. This means that a 100 pair of 10x42 binoculars, even if very well adapted to low light settings, offers a very clear contrast during the day, but a less clear contrast at twilight or dawn. This contrasts with the 500 binoculars (purple curve), which have a much more marked transmission rate with warm colours (the type of colours at sunset), which provides a bright and contrasted observation in low-light conditions. The red curve representing our 900 10x42 binoculars (which have a dielectric treatment) shows an unvarying transmission rate: of close to 97% transmission of the light, whatever the wavelength. This means that, whatever time of day you use the binoculars, you will have marked and relatively neutral contrasts (particularly at twilight when stalking or hide hunting, or when observing wildlife late in the evening).

How do binoculars work?

With our 900 binoculars, the centre of the image that you will be able to see will have a sharpness of detail equivalent to 0.8 arc minute. This definitely won't mean much to you, but 0.8 arc minute represents a higher level of sharpness than the human eye (which has a measurement of one arc minute, which is equivalent to the size of a football located 800m away from your eye). So, this improved resolution in the centre of the image allows the contrast and brightness to be improved.

The frame component of a pair of binoculars

To finish, we’ll tackle a final technical detail. Our 100 and 500 binoculars have a plastic frame, while the 900 binoculars have a magnesium frame. This material, magnesium, is very resistant to shocks, impact and thermal dilations. Thermal dilation is important as you may often leave your binoculars in your car. In summer, when it’s very warm, if you leave your binoculars on the dashboard, for example, the temperature can get very hot. In these situations, plastic unfortunately has the property of melting at high temperatures. Because of this, we recommend that you do not leave your binoculars in your car as the internal anti-fog gas in these binoculars (our level 100 and 500 ones) may escape as the plastic frame melts, which can lead to fog inside your binoculars. Do not expose the 100 and 500 binoculars to large variations in temperature!

How do binoculars work?

Designing binoculars is not easy. There is still a lot of technology behind these products which can seem simple to use. And you, were you aware of these factors before reading this article?