Optic Terms

Optical coatings

When you look at the lenses of a binocular or spotting scope, you’ll notice tints in the glass that are usually purplish/greenish in color. What you are seeing are the anti-reflective coatings that have been put on the lenses. These coatings serve to reduce light reflection and scattering at the air-to-glass surface. When light strikes uncoated glass, a percentage of it (4-5%) is reflected back from the surface, and with 10-16 air-to-glass surfaces in a pair of standard binoculars or a spotting scope, almost 50% of the light passing through uncoated optics would be lost! By applying just one layer of anti-reflection coating, loss due to reflection can be reduced to 2-3%, and by applying multiple layers of coatings, light loss can be reduced to a mere .5% per surface!

Optical coatings are made from certain metallic compounds (most often involving the compound Magnesium Fluoride, but many manufacturers have proprietary coatings whose ingredients are trade secrets) that are vaporized and applied to the optical glass in very thin layers (measured in microns; millionths of a meter) inside a vacuum chamber. The quality and quantity of optical coatings matters a great deal in determining how bright and sharp a binocular or spotting scope will be.

There are some standardized terms concerning the level of coatings applied to binoculars and scopes. With optical coatings, more is better! With more coatings comes increased resolution, contrast, color fidelity, and increased light transmission.
fully coated optics: all air-to-glass surfaces are coated with an anti-reflective coating film. Many modestlypriced binoculars offer fully coated optics and have good but not great image quality.
multi-coated optics: one or more surfaces are coated with multiple anti-reflective coating films. Image quality with multi-coated optics can be quite good, except perhaps in lower light settings.
fully multi-coated optics: all air-to-glass surfaces are coated with multiple anti-reflective coating films. Fully multi-coated optics offer the highest image quality.

Important optical terms

Resolution: The ability of a binocular/spotting scope to separate and distinguish thin lines with clarity. Resolution is essentially the same as image sharpness.

Resolution test: A chart on paper containing a series of sets of lines at progressively smaller spacing and used to ascertain the limiting number of lines per millimeter that a binocular or spotting scope is capable of resolving clearly.

Contrast: The ability to distinguish differences in brightness between light and dark areas of an image. Because we see much of the color spectrum, contrast also refers to the ability to distinguish differences in dimensions of hue, saturation, and brightness or lightness. Optics with superior contrast transmit colors that appear very dense and saturated.

Transmission: The percentage of light that passes through the binocular or spotting scope and reaches the user’s eyes. With modestly priced optics, transmission generally ranges from 85 – 90%. More expensive optics can achieve transmission of 95% or more

Chromatic aberrations: Because different colors move at slightly different wavelengths, they will come to focus at slightly different lengths when they pass through optical glass. The resulting false colorations (seen most often as purplish and greenish ghost images) diminishes resolution and color fidelity. Chromatic aberrations will be negligible with binoculars and scopes that use better optical coatings and/or higher quality glass.

Distortion: The disability of a binocular or spotting scope to deliver an image that is a true-to-scale reproduction of an object. There are principally two types of distortion to be concerned with; barrel distortion (where images bow outward and look bulged), and pincushion distortion (where images bend inward). In both cases, the distortion is due to a poor or compromised optical design and any binocular or scope that exhibits distortion should be passed up.

Astigmatism: The lenses used in a binocular or spotting scope usually have a curved shape, and thus all light rays passing through will not converge on the same focal plane. If this physical reality isn’t remedied in the overall optical design, a binocular or spotting scope will provide images where either the center image or the edge image is in focus, but not both (without refocusing).

Astigmatism cannot be eliminated completely, but it can be kept to a minimum. Users will want to avoid binoculars or spotting scopes that exhibit too much astigmatism, as it cuts into the image quality.

Alignment and collimation: In a binocular or spotting scope, the optical components must, for the best performance, be situated as they were initially designed. Poor manufacturing and/or poor handling of the equipment can cause any or all of the components to become misaligned, resulting in diminished performance.

In a binocular, the optical components (primarily the prisms) in both barrels must be pointing in the exact same direction, known as collimation. Viewing through binoculars that aren’t perfectly collimated (whether they became miscollimated through poor construction or mishandling) can cause great eye strain and fatigue. Porro prism binoculars are much more susceptible to collimation issues than roof prisms.

You can test a binocular for collimation by looking through them at a horizontal line (a door frame at about 15 – 20 feet works very well) and then slowly and carefully pulling the binoculars away from your face so that you can start to see where the two exit pupils intersect. The horizontal lines in each exit pupil should match up correctly. If they do not, they are out of collimation and need repair.


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Optics Terms

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