Buying Awesome Binoculars, Fourth in a Series

by telescope review guide

In my last article I discussed the various prism systems one could expect to find in binoculars. Now I’ll talk about lenses and lens coatings and how they affect the viewing experience.

Chromatic aberration is caused because light of different colors does not bend the same amount when passing between mediums of differing refractive indices such as glass and air.

This prism shows how different wavelengths of light is bent in differing amounts as it passes from air to glass and back to air again.

This prism shows how different wavelengths of light is bent in differing amounts as it passes from air to glass and back to air again.

Blue light, for example, will not focus to the same plane as red light. The effect can create a ring of color around sources of light, and results in a general blurriness to the image. Chromatic aberration is minimised by using an achromatic doublet, or achromat, in which two materials, often crown and flint glass, with differing refractive indices are bonded together to form a single lens. While this reduces the amount of chromatic aberration over a certain range of wavelengths,  it does not produce perfect correction.
This problem can be reduced in several ways. One method is to apply a thin film to the eyepiece element that corrects. The more traditional approach is to eliminate the aberration by using multiple elements of different types of glass and curvature.
An apochromat is a lens or lens system which has even better correction of chromatic aberration, combined with improved correction of spherical aberration. Apochromatic lenses are designed to bring three wavelengths, typically red, green, and blue, into focus in the same plane. Apochromats are much more expensive than achromats.

Antireflection lens coatings reduce the amount of light reflecting off of the lens and allow more light to pass through. Without coatings, up to 50% of the light entering the binoculars can be lost to reflections because of the many glass surfaces within. The more expensive brands will have multiple coatings on all the lenses which will help to give the brightest and clearest images. The most used and least expensive coating is a single-layer of magnesium fluoride but there are also modern broadband multicoatings. Magnesium fluoride reduces reflections from 5% to 1%. Modern lens coatings , such as zinc sulphide or titanium dioxide, consist of complex multi-layers and reflect only 0.25% or less to yield an image with maximum brightness and natural colours. To save money, some optics manufacturers coat only some of the air-to-glass surfaces. Common antireflection coatings often look somewhat bluish, since they reflect slightly more blue light than other visible wavelengths, though green and pink tinged coatings are also used. Some binoculars ruby coatings intended to reduce glare in bright light and improve the contrast between brown and green objects. You should avoid any binocular that uses these coatings because it will perform poorly for astronomical use.

Coating symbols:

Coated (C) – One or more surfaces are coated.
Fully-Coated (FC) – All air-to-glass surfaces are coated but plastic lenses may not be.
Multi-Coated (MC) – One or more surfaces are coated.
Fully Multi-Coated (FMC) – All air-to-glass surfaces are coated.

Binoculars come with two types of focusing mechanisms. Most people opt for the center-focus model, which uses a centrally mounted wheel to adjust both eyepieces at once. There is also a separate adjustment for the right eyepiece, which helps to correct for any difference in near or farsightedness between your eyes.
The second focusing system uses individually focused eyepieces and has no centrally located focusing mechanism. Even though focusing is slower compared to the previous model, binoculars that use individual focus tend to be more rugged and less prone to moisture infiltrations.

Hermetically sealed binoculars filled with dry gas, often nitrogen, will not be susceptible to clouding due to condensation at low temperatures; this will also help to prevent mildew, although air may leak in over a period of years if the binoculars are not properly maintained.

Because binoculars are basically two small telescopes mounted side by side, an error in collimation (optical and mechanical alignment) can lead to numerous problems including eyestrain and double-images. For most binoculars collimation problems are not immediately obvious when you first pick the instrument up and view through it. If after using the binoculars for several minutes your eyes feel uncomfortable as they compensate for the barrel misalignment, most probably the binoculars are out of collimation, which means that the two barrels don’t point in the same direction. This is a serious problem, and you shouldn’t buy those binoculars.

Keep your binoculars in their protective carrying case to prevent dust and grit getting into the mechanism. This can clog up the lubricants and make the controls grind which could eventually seize up. Also avoid knocking them because prisms are often mounted lightly and a bump can misalign one, causing double vision. For the best view keep the front and rear lens surfaces clean with optical cleaning fluid and a soft lint free cloth.

Now that you know everything there is to know about choosing binoculars, I’ll write some articles about just what exactly you can expect to see with the new binoculars that you’ll want to buy with your income tax refund!

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