How Optics Determine the Quality of Binoculars

How Optics Determine the Quality of Binoculars

How Optics Determine the Quality of Binoculars

My love for optical devices dates back to when I was only 10 years old. I still remember the binocular my dad bought me as a birthday gift. In fact, I still keep it – but as a collector.

And ever since my encounter with this optics I have fallen in love with binoculars and optics. I can’t remember how many times I have been blown away by new designs. And today I own one of the best quality binoculars collection.

Well let me not bore with my stories. In this post, I will share with you how the optics determine the quality of the binoculars. I also want to give credit to as I have collected some information from their binoculars buying guide.

Let’s dig in.


The very first part I will deal with is the transmittance. If you need quality binoculars going with high end transmittance is vital. So what do I mean by the transmittance when it comes to optics? Well here is the thing.

In order to use your binocular to view crystal clear images, light has to travel through its optical system. As the light travels through, there is usually a certain percentage of this light that will be lost. The light is lost through a process called absorption and reflection.

This usually happens inside the optical prisms system itself. With this loss of light intensity there is always a reduction on the amount of the original light available upon observation. Meaning that upon exit at the eyepiece the original light varies from usually 50% to about 97%.

It is never again 100%. Now this amount of light in %-age that’s not lost as “white” light goes through optical system is the transmittance.

This figure is however dependent upon quite a number of factors including number and quality of the optical glass used. More importantly it also got so much to do with optical glass elements that have been used in the following:

  • Lenses
  • Prisms
  • Configuration
  • Size of the prisms
  • Collimation of optical system
  • Type and amount of coatings present

What is the Best Transmittance Level?

I want you to understand that for quality image (crystal clear images) you need more transmittance (light). Therefore a higher transmittance level is one of the factors that determine high quality binoculars.

In short a device that has a transmittance level of around or above 90% is a great buy. The same can’t however be said of devices that use lower coatings and glass quality.

Therefore, don’t be surprised when you find better quality images in a 10 X 40 binocular with an exit pupil of 4mm; and a 90% transmittance; as opposed to a 7 X 35 of exit pupil 5mm with 70% transmittance.

How to Determine High Transmittance Levels in a Bino

Let me assure you that most manufacturers won’t display the transmittance levels of their binoculars. However, if they are the transmittance to beat they might. Nonetheless, there are a number of tips you can use to determine high transmittance levels. They include:

  • The use of high end prism glass – BaK-4
  • Use of Fully Multi-Coated lenses
  • Use of Prisms with High Reflectivity Coatings

NOTE: I have just mentioned these points here because I will tackle them in my next sub-topic. I will shed more light on how they work and give an insight on what you should know. So keep reading to find out more.

Vital Optic Features that Determine High Quality Binoculars

Like I have mentioned above, you will find more on the points I mentioned here. Let’s take a look at what we have.

BaK-4, BK-7 and SK-15 Prisms

Modern binoculars use any of the three types of glasses here. These glasses have different qualities. The type used in a binocular will therefore have adverse effect on the image quality. And that’s why price plays a key role.


BK-7 glass is found mostly on low cost binoculars. It is inferior to BAK4, but will still provide good optical touch for some binos.


Unlike the BAK-7 glass, BAK-4 prism glass is of a higher quality. Therefore any device that uses it offers better amounts of clarity and transmittance. Also known as Barium Crown glass, BaK-4 has high refractive index. It also has lower critical angle compared to others which translates to an improved light transmission.


SK-15 is less common. However, it is the glass that strikes a great balance between these two commonly used glasses.

Anti-Reflection Coatings

Most modern binoculars give anti-reflection coatings even if on the air to glass surfaces. The best technology is the use of Fully Multi-Coated lenses. FMC lenses improve/ boost light transmission. Nonetheless, you need to understand a manufacturer’s description of their coatings as no coatings are equal.

FMC is a technology that allows all air to glass surfaces to receive multiple layers of the anti-reflection coatings. This is as opposed to “Fully Coated” or “Multi-Coated” that means a fraction of the surfaces will have coatings. The latter will not match the former in performance.


We have two types of prisms on optic devices. We have the roof and porro prisms.  Between the two, Roof prisms have many advantages over porro. However, it usually has a single surface that won’t provide total internal reflection.

Prisms with High Reflectivity Coatings

Due to this, it’s important to raise the reflectivity of this “dull” surface to get a high end optical performance. An aluminum mirror coating will fix this and improve the reflectivity of 87% to 93%. If not a silver mirror coating can be used.

However, it is the use of a dielectric coating as opposed to the metallic mirror coating that will have the best results. It provides a higher reflectivity say of more than 99% across every visible light spectrum. So when choosing roof prism; make sure there are coatings used to increase light reflectivity.

Extra Low Dispersion Glass (ED Glass)

Low dispersion glass has been used on lenses for quite some time now. However, it is becoming more important to add other elements which have extra-low dispersion property.

These elements if properly added have the capacity to correct any chromatic and also spherical aberrations. Getting an ED glass in your binocular might require you to throw in a few extra dollars. However, it’s worth the trouble.

NOTE: The manufacturing levels differ. So don’t be surprised to get an ED glass on a low-cost bino and end up with chromatic aberrations.

Exit Pupil

The exit pupil is simply the measurement of the exact diameter of the main shaft of light that will come out of the binocular’s eyepiece and directly onto your eye. The exit pupil is a very significant pointer of how well your binocular will perform in the low lit areas. In other words, larger exit pupil will mean more light in and through the eyes.


Getting high quality binoculars is not that simple, I agree. But that doesn’t mean it can’t be done. Simply get your ducks in a row and look at the right pointers. I have given you a bunch and I believe you have found them helpful.

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