Koolertron 12×42 Binoculars

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Koolertron is a trademark of Hong Kong-based Karstone Technology Co., Limited, which distributes consumer and industrial electronics. Recently, the company has been selling binoculars in addition to digital microscopes. I was given these Koolerton binoculars and thought I’d try them out.

Koolertron 12×42 Binoculars

Koolertron 12x42 Binoculars

With a price tag of just around fifty dollars, these optics are very affordable, not to say cheap, but that doesn’t necessarily mean that these binoculars aren’t perfoming.

The binoculars are delivered in a simple box, there is a carry case, neck strap, lens and eyepiece cap, and also a small lens cleaning cloth included in the package.

There are a lot of low-cost binoculars from China that do the job for the occasional “when you need binoculars” situation. Let’s see how the Koolertron 12×42 performs.

Granted, these binos are not high-end top performers, but most users who only use binoculars every once in a while don’t need premium optics that cost thousands of dollars.

Specifications of Koolertron 12×42 Binoculars

Optical Glass HD Glass
Lens Coating Fully Multi-coated
Prism type BaK4
Magnification 12x
Objective diameter 42 mm
Exit pupil 3.5 mm
Eye relief Missing in Specs,
Estimated 12 mm
Field of view 114m/1000m (claimed in specs, but unlikely)
guess: 85m /1000m
Close focus 10 feet
3 meter
Twilight factor 22
Waterproof  No
Weight  15.4 oz/ 440 gr

First Impression of the Koolertron 12×42 Binoculars

Looking at the Koolertron 12×42 and upon a rough inspection, it really doesn’t differ much from the plethora of other 42mm roof prism binoculars available today. After all, there aren’t that many possibilities in construction and design that could immediately catch your eye, except for little things like the ribbing of the rubber armor, design of the eyecups, size and placement of the focus wheel, etc., because ultimately binoculars are just the two side by side connected telescope tubes.

The real difference between cheap and expensive is hidden inside, i.e. in the quality of the processed optical glass parts and the optimal path of the light through the instrument

The Binoculars Housing

The barrels and the single-hinged middle bridge are made out of polycarbonate. Polycarbonate plastics are excellent housing materials for optical devices. It is a material that is long-lasting, durable, and lightweight, in addition, it is corrosion-resistant and not subject to thermal expansion.

The binoculars are encased in a rubber armor that provides grip and also protection from bumps.

Dimensions and Weight

15 ounces

As far as the outer dimensions of the Koolertron 12×42 are concerned, the instrument appears rather compact. The weight is also surprisingly lightweight at just 15.4 ounces.

It’s nice that it weighs so little, but conversely, that means that savings have been made on the inner workings, on the lenses and prisms. we will see that when we cut open the instrument and look inside.

Tripod Adapter

For handheld binoculars, 12x magnification is a lot, so prolonged observation can tire your arms, making it harder to keep a steady hand, and blur your vision.  So it is nice to see that the binoculars also have a connecting thread under the screw cap on the middle bridge to which you can attach a tripod adapter.

Waterproof Fog Proof

This is the weak point of this budget device. The specifications only say water-resistant, which means that the instrument can withstand raindrops, but there is a problem when it gets really wet.

You’ll notice that when looking at the front lens and turning the focus wheel, you can see that the lens is moving back and forth. In the case of waterproof binoculars, an outer pane with an o-ring is usually fitted to create the waterproof seal.

The Mechanics

Focus Wheel

The large focus wheel at the top of the middle hinge is big enough for easy grip and can be easily turned with your fingers. It moves smoothly and evenly, from near to far takes about two turns.

Middle Bridge & Hinge

The middle bridge does not make these binoculars significantly different from standard binoculars, it hinges well and allows perfect adjustment to the individual pupil distance.

But any binocular manufacturer should be able to build binoculars in which the hinge pivots properly and ensures proper collimation of the optical axis at any angle. Unfortunately, this is not always the case and images that are not properly merged are a not too uncommon problem.

Eyecups and Diopter Compensation

A diopter setting is a feature that should be standard for every pair of binoculars. Of course, the Kooltron has one too. The right eyepiece is equipped with a simple +/- fine adjustment. Which diopter values are involved is not specified further. It was sufficient for me, but I only have a difference between the eyes of 0.25 dioptres, the marking on the adjustment wheel was only slightly in the negative range.

Use this eyecup setting for the best viewing

The eyecups on this model can be twisted out by about half an inch. They noticeably snap into place. First in only about 5mm, then in about 8mm, and finally in the maximum position. The two further settings do no good.

The eye relief is quite short, so the ideal setting is on the 5mm notch. Here you have the maximum field of view, the vignetting that you notice on the edge with completely retracted eyecups is gone. If you rotate the eyecups to the farther positions, you will be out of the eyepoint and the observed image will be smaller than on the 5 mm position.

Optical Performance

The optical performance shows whether a pair of binoculars is useful and promises fun when using it or whether you are more annoyed about the instrument. Only considering the magnification and lens diameter says little about the instrument, the optical glasses used, their material properties, and the coordination of these components are decisive for good optical performance

12×42 sounds good at first, but when you look through the binoculars you quickly realize that that can’t be everything. What is immediately striking is the high magnification and the fairly bright image in clear, in natural colors.

Personally, I don’t like the narrow field of view that much. I prefer binoculars with 8 to 10x magnification because these are much more versatile and can truly be described as all-around binoculars.

Lens Coating

The manufacturer advertises in the product description that the lenses are fully multi-coated.

If you hold the lens so that the light is reflected in it, you can clearly see a faint greenish shimmering reflection on the lens. Looking straight into the tube, the view is clear and there are no discernible color deviations.

If you point the binoculars at a white house wall in the midday sun, it appears in the same shade of white as if you were looking at it with the naked eye.


According to the specifications, the binoculars have BaK-4 prisms. This is clearly evident when holding the binoculars against the bright sky and looking at the exit pupil. It’s nice and round.

Unfortunately, there is no indication in the specifications whether the prisms are phase-corrected and whether they are vapor coated or die-electrically-coated. But if you take into account the super low price, you can assume a simple aluminum vapor coating. The simple coating of the prisms is also evident in the optical performance, the edge blur, the slight color fringes, and the not-too-impressive contrast.

Eye Relief

I already briefly mentioned the eye relief above. Unfortunately, one looks in vain in the specifications and finds no exact numerical value. It’s quite short, I guess it’s about 10mm.

However, most normal-sighted people will be fine with this model, but if you must wear your glasses when viewing, these binoculars are not for you.

Close Focus

When testing at close range, I was able to focus on objects from about 9 feet away.

Unfortunately, the manufacturer forgot to include this information in the product specification.


The images of the tubes merge well. No double images either at close range or at far distances.

Edge sharpness and Edge Blurring

The Kooletron produces sharp images in the center of the field of view, but from halfway to the edge, edge blur begins to increase sharply. Considering the price these binoculars cost, you shouldn’t expect impressive, consistent image sharpness.

The rather weak depth of field of this optics leaves much to be desired. Frequent refocusing is necessary even when looking from one object to another that doesn’t seem to be too far apart.


Chromatic Aberration / Color Fringing

Chromatic aberration naturally occurs in this model at the light-dark contrast. But it’s really not too bad and with normal observation, you will hardly notice it. That’s amazing, I think the coating of the prisms isn’t that bad.

Color fidelity

Colorful objects appear through the binoculars in the same hue as seen with the naked eye.

Pros of the Koolertron 12×42 Binoculars

  • Extremely low purchase price
  • Adequate optical performance
  • Low weight
  • Accessories: eyepiece and lens covers, cleaning cloth, padded carry case, neck-straps


  • Important specs are not listed in the product description
  • Short Eye Relief
  • Not waterproof

Using The Koolertron 12×42 Binoculars

12x magnification is really a lot for handheld binoculars. While this has the advantage of being able to see things that are far away in detail, it has the disadvantage of a slightly narrower field of view compared to 10x or 8x magnification. Also, it can be a little tedious to hold the instrument still when you get tired during a long observation and your hand becomes a little shaky.

Focusing works well and quite quickly, but this is also necessary as the shallow depth of field requires constant adjustment of the focus.

With casual observation out in nature or in the city, you can’t really complain. The images are sharp in the center and the observation is true to color. That there is a lot to be desired in terms of edge sharpness of the images, depth of field, and also chromatic aberration is not surprising given the low purchase price.

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Wrap up Koolertron 12×42 Binoculars

The Koolertron 12×42 roof prism binoculars are a great bargain for casual users. With its 12x magnification, it brings distant objects up close so one can see wildlife or birds in detail from a safe distance without being seen or spooking them off. The 42mm lens diameter captures enough light to provide a bright image even in poor light conditions.

The low weight of the binoculars and the fairly compact pack size is practical so that you can easily stow them in your backpack or bag and have them quickly to hand when you need them. A straightforward simple binocular that performs satisfactorily, the price-performance ratio is really good.

What the manufacturer absolutely needs to improve is the datasheet, which requires more precise specifications. Important data such as near focus setting, eye relief, and type of prism coating are not given at all and some are even wrong, such as a field of view of 114 meters/1000m, which will hardly be correct in a 12x magnification. If I aim at objects on the horizon with binoculars and estimate the degrees between the two objects, then the field of view seems to be about 90 m.

A disadvantage is the rather low eye relief, which is sufficient for people with normal vision, but the instrument is useless for people who always need to wear glasses. The eyecups aren’t really well thought out either, they twist out way too far considering the short eye relief, so remember not to twist more than halfway out otherwise the field of view will shrink.

Unfortunately, the instrument is not waterproof, so you must take extra care in extreme weather. Here is a budget binocular from UScamel that is waterproof. If you spend a lot of time outdoors on birding excursions hunting trips, or hiking and trekking, you always have to expect the weather to suddenly change and get really bad. So it is better if you pay attention to a waterproof and fog-proof rating when buying binoculars.

In conclusion, even if the optical performance is not spectacular, considering the incredibly low price there is nothing to complain about. If you need more and better optical performance, like more depth of field, more edge sharpness, and waterproof and fog proof features, you have to spend a little more money.

See the Koolertron 12×42 Binoculars on: koolertron.com