Beginning Optics

Basic ideas, general design, function, and features of binoculars and spotting scopes.

How do binoculars work?

Allbinocularanatomy binoculars, regardless of their size and shape, function in the same, straightforward way: Light comes to and moves through the objective lenses.

  1. Light then travels through the prisms (which correct the image orientation in all directions; up-down, left-right).
  2. Finally, light moves through the eyepieces (which magnify the images) and then on to the user’s eyes.

What determines image quality?

Optical glass – The quality of optical glass that is used in binoculars will make a difference in how bright, sharp, and colorful the view will be. Quality binoculars use dense optical glass that is painstakingly designed, shaped, and polished to eliminate flaws. The more sophisticated the glass and techniques employed in its design, the better the images.

Anti-reflection coatings – Binocular lenses are coated with anti-reflection coatings to eliminate internal reflections and light scattering, reduce glare and produce sharper images with more detail. The type of coatings and the number of coatings applied to the binoculars matter tremendously to how brilliant and crisp the view will be.

Exit pupil – The exit pupil is the beam of light that exits each eyepiece of the binocular and enters the users’ eyes. The larger the exit pupil, the brighter and more superior the image will appear, especially under low light conditions (when comparing optics of similar quality). The exit pupil is measured in millimeters, and is calculated by dividing the objective lens by the magnification. An 8×42 binocular, for example, has a 5.25mm exit pupil (42÷8=5.25).

What do the numbers mean?

When you look at a pair of binoculars, you’ll notice a few numbers printed on the body of the binoculars, such as 8×42 (read as 8 by 42), 7.2 degrees (378 @ 1000 yds.). What do these numbers mean? What do they refer to?

binoculardesign21. Magnification
With a pair of 8×42 binoculars, as in our example, the first number, 8 (often expressed as 8x), refers to the magnification the binoculars provide, or how many times larger an object will appear. Binoculars vary in magnification from 4x up to 12x and even higher, but 8x and 10x are most common.

Higher magnification is not necessarily better. As magnification increases, users may have trouble holding the binoculars steady, causing the image to become blurry. An increase in magnification will also generally cause a decrease in image brightness and clarity. 7x or 8x magnification is considered adequate for general observation, while 10x is a great choice for viewing at greater distances.

2. Objective lens size
The second number in our example binocular, 42, refers to the diameter of the objective lens (the lens farthest from your eyes) in millimeters. Objective lenses vary in size from 15mm to 50mm and beyond (binoculars with objective lenses larger than 50mm are often used for astronomy and surveillance and require a tripod).

The size of the objective lens determines how much light the binoculars can receive and hence how bright and clear the resulting images can be. The size of the objective lens also affects how large or small a pair of binoculars will be. Let your needs and desires help you decide what size objective lenses are right for you. If you use your binoculars only during the brightest times of day or in well-lit areas, then smaller objective lenses (say, under 25mm) will do just fine. If, however, you want the brightest possible image and will be using your binoculars during near-dark conditions (such as at dawn, dusk, or in heavy forest cover), you’ll want to choose larger objective lenses, from 35mm to 50mm.

The greatest factor in determining the weight of a binocular is its objective lens size; the larger the lenses, the heavier the binoculars will be. Again, let your desires dictate what weight is comfortable for you. Compact binoculars can weigh between a few ounces to under a pound, while modern full-size binoculars will weigh from twenty ounces to around two pounds.

3. Field of view
The final number that appears on the binocular body, 7.2 degrees (378 feet @ 1000 yds.), is the field of view. The field of view is the widest dimension from left to right that you can see when looking through the binoculars. This specification is usually measured either in linear feet at a distance of 1000 yards, or in angular degrees. Field of view specifications are not always disclosed on the binocular body, but this measurement is a critical specification for many binocular users. A wider field of view is desirable for many reasons, including but not limited to: following fast moving action, and finding birds and animals in denser backgrounds (grasslands, woodlands, etc.). Note that as magnification increases, the field of view often narrows (sometimes considerably).

Other useful specifications

The following specifications and definitions will aid in your understanding of how binoculars can function best for you and provide you with the maximum benefit in your hobby.

Eye relief

The term eye relief refers to the furthest distance behind the binoculars’ eyepieces at which the whole field of view can be attained, and is measured in millimeters.

The eye relief measurement is of great importance to those that must wear eyeglasses/sunglasses while looking through binoculars, but is also important to anyone planning to use binoculars for long stretches of time.

Binoculars with long eye relief will satisfy the above considerations, and will have an eye relief measurement of at least 15mm.

Close focus
The minimum distance to which a pair of binoculars can be focused is called its close focus. Many bird, butterfly and dragonfly watchers desire binoculars that will focus down to 10 feet or less.

Binoculars that effectively keep out the elements will inevitably last much longer and keep you satisfied. Many binoculars offer something in the way of weather resistance, but for the utmost guarantee, choose one that is labeled as waterproof and fog-proof.

Waterproof / fog-proof binoculars are sealed with O-rings to keep out moisture, dust and debris. The inside of the binoculars is then purged of its atmosphere, which is replaced with an inert gas that has no moisture content. This process, called Nitrogen-purging, ensures that the binoculars will not fog internally from high humidity or altitude changes.

A manufacturer’s included warranty ought to be considered a feature of the binoculars, especially if you plan to get a lot of use out of them in the outdoors where anything can and usually does happen. Many manufacturers offer a warranty limited only to initial defects, meaning you’re covered if you notice something wrong with your binoculars during general day-to-day use. Limited warranties do not protect you if anything accidental happens in the general course of using your optics.

More progressive warranties cover the binoculars in literally any situation, regardless of what happened or who is at fault. These “no-fault” warranties offer the ultimate in customer service and protection.

How do spotting scopes work?

A spotting scope functions essentially the same way a binocular does:

  1. Light is gathered and moves through the objective lens of the scope.
  2. Light moves through the prisms (which correct the image orientation in all directions; up-down, left-right).
  3. Light moves through the eyepiece (which magnifies the image) and then on to the user’s eye.

Spotting scope specifications

Spotting scopes are essentially small telescopes designed primarily for land viewing at longer distances. A spotting scope features greater magnifications and a larger objective lens than those offered with binoculars, and as such requires a tripod to be used effectively.

Spotting scopes are generally offered in two sizes, relating to the size of their objective lens; 60mm and 80mm.

  • 60mm scopes are fairly portable and compact, and will offer good image quality at magnifications up to 45x.
  • 80mm scopes will be much brighter than a 60mm scope but will also generally be heavier and potentially bulkier. An 80mm scope will deliver very good image quality at up to 60x magnification.

Spotting scopes are often made available in two body styles, a straight-through design (where the eyepiece is in-line with the objective lens) and an angled design (where the eyepiece is set at a 45-degree angle). One design is not better than the other, but each design does offer some distinct advantages.

As with binoculars, there are other specifications (such as eye relief, weatherproofing, warranty, etc.) that you may want to think about. Eyeglass wearers should look for scopes with at least 15mm of eye relief, and if you plan on using your equipment in inclement weather or around water, you’ll want to consider a scope that is fully waterproof. As with binoculars. a scope that carries a more progressive warranty will offer more piece of mind when out in the field.

What determines image quality?

Most spotting scopes use a Porro prism design that offers a rich three-dimensional view with good image quality. Similar to binoculars, spotting scope image quality is derived from the types of optical glass and optical coatings that are employed in its design. The better the glass and optical coatings, the better the image quality.

Some spotting scopes are offered in two different versions of glass; a “standard” version, and a “high-grade” version. The standard versions employ regular optical glass in their design, and generally offer good to very good image quality. The high-grade versions make use of more exotic (and more expensive) glass types that deliver heightened resolution and color. Consider the high-grade versions (if available) if you desire the best possible image in all lighting conditions.

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Optics Terms

Beginning Optics

Advanced Optics