How Optics Work

What are binoculars? What are spotting scopes? And how do they work? What are some of the important differences between binoculars and spotting scopes? What do I need to know in order to tell which is best for me? By briefly covering these topics, we hope you’ll be better able to select the binocular or scope that will be right for you. By clicking on any of the words in bold blue text on this page, you will be taken to our glossary, where you will find an explanation of the technical terms used here.


We birders owe so much to so many! It might be said that we are closer to the birds because we stand upon the shoulders of giants: Galileo, Kepler, Scheiner, Huygens, Descartes, and Newton. And all because of the wondrous properties of light: multiple wavelengths that are refracted (bent) when they meet media of different densities; air to water, air to glass, air to the lenses in our own eyes. 

All binoculars for birding, and all of the spotting “scopes” we sell here (though not all on the market), are specialized types of refracting telescopes (telescopes in which the light-gathering objective is a lens rather than a parabolic mirror). The earliest telescopes were refracting telescopes and appeared in the late 16th and early 17th centuries. They typically consisted of a plano-convex [i.e. one side flat, the other curved outward: I) ] objective lens at the far end of a tube, and a plano-concave [thus: I( ] ocular lens in a smaller tube that could be moved in and out within the larger tube for focusing. Light striking the glass is bent or refracted as it passes through (some is reflected off the surface). The ocular lens magnifies the image produced by the objective lens. This lens combination yielded an erect (right-side-up) image, and could be used for either astronomical or terrestrial viewing. Galileo’s first telescope (which he made in 1609) was just three power (3x), but by later in the same year he had made an 8x and then a 20x instrument; these were 30 to 40 inches long. 


The magnification of a simple telescope is determined by the ratio of the focal lengths (distance from the lens to the point at which the image produced is focused) of the objective and ocular lenses, e.g. an ocular with a 10-inch focal length divided into an objective with a 100-inch focal length yields ten power. For early lens makers, who lacked the knowledge and skills to make a lens of uniform curvature, minimizing optical defects in the glass meant minimizing the curvature of the lenses. So increased power meant increasing the focal length of the objectives. This was pursued to impractical limits. In 1656 Christiaan Huygens had a 100x telescope that was 23 feet long! Such tubes reached a maximum of about 140 feet, after which astronomers tried attaching objective lenses to buildings and oculars to stands on the ground. Needless to say, aiming such instruments presented quite a challenge. 


Sir Isaac Newton made the first reflecting telescope in the 1670s. Simple Newtonian reflecting telescopes consist of a silvered parabolic mirror at the base or bottom of a tube (the opposite of a clear glass refracting lens) which focuses the image on a small diagonal mirror near the upper end of the telescope and thence out through a hole in the side of the tube through an ocular lens. At that time making parabolic mirrors was even more challenging than lens-making. Not until the mid-18th century with the work of James Short and William Hershel did this type of telescope really come into its own. All of today’s large astronomical telescopes are reflectors; the largest currently in operation is the 10.4-meter (34.12-foot) Gran Telescopio Canarias in the Canary Islands. Note that this is the diameter of the mirror, not the length of the telescope! European scientists have recently approved funding for a 42-meter (137.8-foot) mirror which will be composed of 906 hexagonal segments, each 1.45-meters wide.

Such telescopic wonders are just the thing for viewing nebulous objects thousands of light years away. Birders, of course, can’t hope to follow tree-top warblers with a Newtonian reflecting telescope, and you certainly wouldn’t want to lug it down the beach. So except for the celebrated Questar, the optics we use in birding followed the path of refracting scopes. 


Johannes Kepler demonstrated that telescopes could also be made by combining a convex objective and convex ocular. This produces an inverted image; adding a third convex lens erects the image again. Christoph Scheiner experimented with various combinations and found that a double convex objective [thus: () ], while producing an inverted image, was much brighter and gave a much wider field of view than the plano-convex objective. But in those years, the still-poor quality of glass meant that each added lens increased optical distortions. (To astronomers, “right-side-up” is meaningless; even today, star-gazers eschew telescopes with the additional lenses or prisms needed to produce an upright image.)

From a birder’s (or astronomer’s, or artillery commander’s) viewpoint, all early telescopes suffered from several serious flaws. Because the glass was of such poor quality—full of tiny bubbles, with a greenish cast due to its iron content, poorly ground around the edges and well-shaped only in the center—the objective had to be stopped down with donut-shaped rings inside the telescope tube. This yielded a terribly narrow field of view, about a quarter of a degree. Imagine being able to see only a quarter of the full moon! Compare this to today’s binocular field of six to eight degrees. But how did we get to the wide field, crisp views we now enjoy?

Field of View

Figure 1: Field of View

Late in the 17th century, the Galilean telescope was replaced for terrestrial uses (where an upright image is desirable) by a telescope which had four convex lenses: objective, a field lens (in front of the objective lens) which further enlarged the field of view, an erector lens and the ocular.

Another theoretical development came from Sir Isaac Newton in 1672, when he demonstrated that white light is a mixture of colored light of different refrangibility. Every color, it was learned, had its own degree of refraction. Alas for early optics; any curved lens decomposes white light into the colors of the spectrum, each of which comes into focus at a slightly different point along the optical axis. The result is known as chromatic aberration, and is often seen as “color fringing” in poorly made binoculars or telescopes. It is most easily seen when viewing a discrete light source such as a planet or bright star, which may appear to have multiple concentric halos. (In birding optics, it is frequently notable at higher powers in scopes with regular glass.) Since this aberration can occur at every lens surface, the more elements the greater the difficulty in correcting chromatic aberration. Also, not all light passes through a lens; again at each surface, some is reflected rather that refracted.

Chromatic Aberration Star Test

Figure 2: Chromatic Aberration  Figure 3: Star Test 

From the mid-18th century through today, we have enjoyed a steady progress in practical advances in glassmaking and lens grinding, and theoretical advances in optical science. Hence, the constraints in objective lens sizes, lens curvature and bubble-filled, green glass have been gradually overcome.

The first binocular instruments—field glasses—were simply two Galilean telescopes joined by braces; later, hinges were added so that the interpupillary distance could be adjusted. These instruments found practical uses for more than 75 years, especially in the military. Low power “opera glasses” were reasonably short, but higher power instruments were commonly 12 to 24 inches in length.

Whether field glasses or true binoculars, the oculars must bring all the wavelengths of light to a single focus; this circular image, appearing suspended above the ocular, is called the exit pupil. The apparent distance between the ocular lens and the exit pupil is called the eye relief. This is critical for eyeglass wearers, as, if it is too short they will see only part of the field of view. Even non-eyeglass wearers may experience vignetting if the interpupillary distance and eyecups cannot be adjusted to bring the exit pupil to their eye.

Eye Relief

Figure 4: Eye Relief


With the development of prisms—first the familiar Porro prisms in the late 19th century, then penta prisms (briefly in vogue before and after 1900), and then roof prisms—the length of both binoculars and scopes has been significantly shortened. By bouncing the image around many internal corners, the light path can still be long (allowing for increased magnification) while the optical instrument itself can be miraculously compact.

Simple Porro Prism Binoculars

Figure 5: Simple Porro Prism Binoculars

As our optics become more compact, they also become more complex. Today’s glass manufacturing is a true art and science. No manufacturer would accept even a single visible bubble in their optical glass (though some cheap glass may be a little milky), or obvious color in the glass itself. Porro prisms must have highly reflective and permanent mirrors, while roof prisms must be exactingly ground of only the finest glass.

There have been improvements in non-optical construction as well. External focusing, wherein the ocular lenses are moved away from the objective lenses (actually making the scope or binocular physically longer or shorter), is now found in only low-cost binoculars. In most binoculars and spotting scopes today focusing is wholly internal, making construction of waterproof optics feasible. Lastly, the tubes have changed from bronze or steel to lighter-weight aluminum or even lighter magnesium-alloys or poly-carbonate.


Some of today’s top optics bear little resemblance to the simple Porro prism design shown above. An angled scope may have seven to thirteen lens elements and prisms, precision-ground and with as many as nine carefully formulated and applied chemical coatings. Multiple coatings are needed as each increases transmission of specific wavelengths of light. Thirty years ago coatings were commonly applied only to the outer, “air-to-glass” surfaces. Today, most top shelf optics are “fully multi-coated”, meaning multiple coatings are applied to all lens and prism surfaces. Additional and different coatings, well-applied, increase the total light transmission and minimize chromatic aberration.

It is at higher magnifications that one benefits most from the high-end glass (called high density (HD), extra-low dispersion (ED) or apochromatic (APO) by various manufacturers) offered in the objectives of the flagship spotting scope and binocular models. The performance of a manufacturer’s standard glass scope versus a high-end scope often appears indistinguishable at 20x. But crank it up to 40x, and chromatic aberration may show up in the standard-glass scope as color-fringing at the edges of feathers, making an Alder Flycatcher, for example, both less sharp and disconcertingly colorful. HD / ED / APO /FL / Prominar® versions provide feather-edge sharpness with true-to-life colors. Imagine what Galileo would have thought of the latest flagship spotting scopes! The apochromatic objectives and anti-reflective coatings of today’s top-of-the-line binoculars would have sent Newton into paroxysms of joy.


At the beginning we said it was “all about light”. Now we’re about to contradict ourselves. Thanks to three- and-a-half centuries of progress in optics and glass-making, all of the top makers and models offer such superb optical quality that, quite often, deciding on the binocular or scope that is right for you will rest on mechanical and ergonomic differences. How does it fit your hands? Your face? Where does your focusing finger rest most comfortably? We are happy to listen to your needs and help you narrow the range. But you, not we and not your friends, are the final arbiter.