The best binoculars are hard to find – especially if you aren’t much aware of them. You need to do more research, find more tips, and read a lot before you buy. But is everything that they are telling you true?
In our previous blog post, we have discussed how to determine quality binoculars. Today we are going to discuss some of the binoculars hype that’s costing you money. They are things that you wish you knew before purchasing your next binoculars. Hang in there and let’s take a ride.
Binocular’s Cover-Plate
So, you have seen some cool things printed on your next binocular’s cover plate. They are enticing…right? They are Fully Multi-Coated (FMC), BAK4 prisms, high magnification power, large objective lens, etc.
Well, don’t be deceived by those specifications. Forget even the internet forums and leave alone the cool binocular’s cover-plates.
Why Do I Say So?
First, there are very many important things that the cool binocular’s cover-plates or the vendors don’t fill you in. Especially, when it comes to Fully Multi-Coated (FMC) lenses or the BAK4 prisms such as these:
- Edge distortion
- Prism housings
- Smoothness of focus
- Field curvature
- Size of fully illuminated field of view
- Spherical aberration
- Quality internal light-baffling
- Type and quality of the eyepieces
- Crispness of focus
- Vignetting
- Construction/Build quality
- Chromatic aberration
So, What about Coatings and Prisms?
Fully Multi-Coated Optics
When the coating is done properly, multi-coated optics will transmit more light compared to the single-coated or uncoated optics. It will be much more when all the lenses are coated – as in fully multi-coated optics.
Application
To achieve an ideal transmission of light, the coatings must be applied evenly and to the right thickness. In case they are unevenly applied or if the application is even but with the wrong thickness, it’s as good as the regular toy bino.
How to Identify
Uneven-application of coatings might look “blotchy” but not always. Worse off is that without the proper testing tools, you won’t be able to find out if the right thicknesses are ideal. In short, you depend on trust.
You are going with quality control and hoping that you get the right optics. But quality control is also relative and while many people complain about the quality this is the truth:
To improve the quality and consistency of optics production needs more time. It also needs better glass selection, more training and assembling time, and more resources. This way, a higher quality binocular can be designed. However, the final cost will be very high and most people will shy off again.
The Reality
To say the truth, we are at an age where the “accountants are in charge” and not the engineers. No more input in products. We concentrate more on the cost of production, affordability to the majority, and that’s just it.
We give a budget before the optic is designed. We don’t design a binocular first and attach a price tag to it later. No. Why? Because we fear the end cost might be too high for anyone to afford and low sales mean losses.
Fully Multi-Coated: What It Really Mean Today
In short, the term “Fully Multi-Coated” is also relative. Just like “affordable” FMC can now mean that all the glass-to-air surfaces of the objectives and eyepieces lenses and the transmissive prism faces all have the 7 layer interference coatings with proper application and thickness.
Or it can also mean that only the glass-to-air surfaces have more than a layer interference coating. So, yes, you never quite really know what you are buying unless the manufacture gives more details.
Magnification Power and Aperture
A”10×50″ that’s printed on a binos cover plate will mean 10× magnification power and 50mm objective lens diameter/ aperture.
So it’s logical to assume that when you measure your exit pupil, it will be 5mm (50mm ÷10)? I measured a few models and I was disappointed. I found 4.2 mm instead. It means that your binocular is effectively the 10×42.
But why say it’s a 50mm which transmits 1½ times much light than the 42mm? Here is what I think.
When the objective lenses are ground poorly, the edges are affected mostly. So, the light coming from the lens edges produces distorted images. In order for these distorted images not to occur the manufacturers use a diaphragm in between the objective lenses and the eyepieces. The diaphragms stop the light coming from the edges to the eyepiece. This makes a 50mm objective lens to a 42mm or even lower. Knowing only the objective lens diameter, therefore, does not tell us how much light a binocular can gather to produce a bright image.
Moreover, if they stop the aperture down to 42mm they might just escape with the use of smaller prisms. The smaller prisms often require a smaller housing. It also demands lesser fixing.
In addition to that, going down to 42mm will also increase the optics effective focal ratio to about f/4.2 from about f/3.5, which demands lower eyepiece quality. It also boosts the color correction and the edge performance.
In short, leaving as it were will cost more to design a top-rated bino. Stepping it down is the shortcut to getting what they need. Less cost, lower quality optics, and main goal achieved. It is that bad.
BaK4 Prisms
Bak4 glass originates from Germany. The BaK in the BaK4 stands for “BaritleichKron” which is “Barium Crown” in German.
BaK4 is often used in high-end binoculars prisms. Its advantage is that it has higher refractive index over the BK7 glass.
In short, BaK4 is highly effective when used in any binocular to provide more light. It enables additional light to pass from the border of your field of view, through the prisms, and eventually into the eyepieces.
This action provides a brighter edge of field. However, there won’t be any effort to make the middle of the field brighter. Nonetheless, people have seen it as desirable because of this simple analogy.
What You Should Know
There are international industrial optics standards that determine what type of BaK4 glass that should be used. For example, the international BaK4 standard designation is 569561 with the first three digits telling the refractive index (1.569) and the remaining three telling you the Abbé number (56.1).
| Glass Type | Refractive Index | Critical Angle | Dispersion |
| Schott BaK4 | 1.5688 | 39.6° | – 0.0523μm -1 |
| Chinese BAK4 | 1.5525 | 40.1° | – 0.0452 μm -1 |
| Scott BK7 | 1.5168 | 41.2° | – 0.0418 μm -1 |
The Abbe indicates the amount of light that will disperse into its constituent colors. The higher your Abbé number the lesser the dispersion.
Even so, the “BAK4” glass on the Chinese optics is not the original Schott BaK4. The Chinese binoculars use phosphate crown glass and not the Barium Crown. Phosphate crown has lower refractive index, dispersion, and abilities compared to the Schott BaK4.
- What the Bak4 on The Cover Plate Don’t Say
- The Bak4 prints on bino covers don’t tell you the following:
- That if your prisms are under-sized they cut some light out.
- What precision the prism’s flat surfaces are polished too.
- If the prisms have grooved hypotenuses that reduce the spurious reflections
- If the sides of the prism are blackened, shielded to prevent the entry of non-imaging light
- The mechanism used to secure the prisms into the bino housing.
No one tells all these things. Some of them, you might never find a solution to. Nonetheless, they are costing you money. You will end up with a good optic device on the cover and lower quality in reality – during use. My suggestion is simple – start learning more about optics now.