Optics University

The technical side of optics

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Most lenses form images that are slightly out of focus, even at the location where a sharp image (focal point) is supposed to be formed. The lens defects causing this lack of focus are called aberrations. Some aberrations result from improper lens construction and flaws in the materials used to construct the lens. Others, such as spherical and chromatic aberrations, result from inherent refractive properties of light passing through the lens. Chromatic aberrations are specifically caused by the difference in the degree to which different colors of light are refracted. As a side note, astigmatism is a form of aberration in the eye.

Close Focus

Close Focus is an important binocular specification primarily for birding enthusiasts and nature observers. Every pair of binoculars can focus out at the farthest field of view. The challenge is to be able to obtain a sharp clear focus of your subject at short distances. There are many opportunities for birders to experience a bird flittering around and actually landing on a branch very close by. This is a great opportunity to see stunning detail, color, and texture of the bird being studied and having a pair of binoculars that allows a close focus can make that visual experience even more memorable.

Astronomy, hunting, stadium events, marine, or horse and auto racing enthusiasts will seldom need a critical close focus specification.

Diopter Adjustment

Quality binoculars will have a diopter adjustment on one barrel that allows a user to balance the focus when viewing through the binoculars without the aid of eyeglasses or contacts. For a person that is using corrective lenses, or has 20/20 vision setting the diopter adjustment to zero would be appropriate. How to set the diopter adjustment for your eyes: This is necessary when you intend to use the binoculars without the use of corrective lenses. Start with the diopter adjustment at the zero mark. Close the eye that would be looking through the barrel with the diopter adjustment- typically your right eye. Focus the binocular on some distant object while looking only with the left eye. Now close that eye and open the right eye on the diopter barrel. Using only the diopter adjustment, simply adjust it until the object is in focus. Now enjoy your view. Be sure to check that setting, as that is where you will always set it if the diopter happens to get adjusted or changed by others.

Eye Relief

Eye relief is defined as the distance in millimeters that the eye can be away from the ocular lens and still have a full unviginetted view of the image being viewed.  This is an important specification for users of both binoculars and rifle scopes for different reasons. Binocular users adjust the eye relief accordingly to their need to wear eyeglasses. Binocular manufacturers incorporate eye cups that typically adjust in or out to meet the needs of the user. Someone wearing glasses would adjust them in whereas someone without glasses would adjust them out. Eye relief in a rifle scope helps to insure that the user is a safe distance from the scope so that during recoil there is no impact of the scope to the face or skull. 

Exit Pupil

Imagine a beam of light exiting the ocular lenses (eyepiece) of a binoculars. The diameter ( in millimeters) of that beam of light is called the exit pupil and is calculated by dividing the objective lens size by the magnification. The size of the exit pupil should be larger than a persons pupil diameter, which of course varies depending upon the ambient lighting level. If the exit pupil is smaller than the pupil size the viewer will see a dimmer viginetted view. This will occur most frequently in low light situations when using compact binoculars. You see, in low light situations your pupils are dilated to nearly 7mm but if the exit pupil is small, such as a 8x24mm compact binoculars- which has a 3mm exit pupil (24 divided by 8) that is a typical situation which results in a undesirable view. The only exception to this is astronomy. In this case the comparison is usually with binoculars that have the same objective lens size but with different magnifications. The larger magnification will produce a smaller exit pupil. However in astronomy, a dimmer/darker background is beneficial as there will be a noticeable increase in contrast with the object that is being viewed, such as a star or other heavenly body.

Field of View

The field of view is a specification that defines how wide the view is at a set distance, such as 1,000 yards. The field of view varies with the magnification. The higher the magnification, the smaller is the field of view. This fact of physics applies to all optics. 

Glass Quality

The quality of the glass which the prisms and lenses are made of will make a profound difference to the image quality of the objects in the view. Standard glass will perform satisfactorily in situations where the lighting is adequate. However in low light conditions, high quality glass will provide the best optical performance. This is the one of the primary features that is the reason for significant cost differential between different sports optics of the same specification. The other is lens coatings.

Interpupillary Distance

Interpupillary distance is a measurement of the distance between a persons pupils. This measurement varies by person and is typically within the range of 64 and 72 millimeters for adults. Most binoculars are adjustable within that range, however they may not fit properly for children. Before purchasing binoculars for children be sure that you investigate interpupillary distance, usage, and price. Once again, compact binoculars are not designed for children.  

Lens Coatings

Lens coatings are necessary to insure that stray light is not reflected away or out of the optic. Manufacturers have invested heavily in coating technologies to help insure that you will experience a truly bright and high contrast image when viewed through quality optics. The whole idea of lens coatings can be very confusing to the uninitiated. Historically there were generally four different coating specifications, which included:
  • Coated optics
  • Fully coated optics
  • Multi-coated optics, and
  • Fully multi-coated optics
Without any fore knowledge about coatings, after reading a sales brochure touting "fully coated optics", one might come to the incorrect conclusion that is the top of the line. However that is not so.  To better understand coatings you need to understand what each means. Every lens will have two reflective surfaces, obviously the front and the back.
  • Coated optics-  a single layer on at least one lens surface, typically the outside surface.
  • Fully coated optics-  a single layer on all surfaces of a lens
  • Multi-coated optics-  multiple layers on at least one lens surface
  • Fully multi-coated optics- multiple layers on all surfaces of a lens
Multicoatings are more efficient than single coatings. The type of coating used in consumer optics is typically a dielectric coating such as magnesium fluoride. Manufacturers establish different coating thicknesses, composition, and number of layers to tailor the transmissivity and reflectivity of the lens. Some manufacturers will also have a phase correction coating applied to the roof prisms used in binoculars to insure maximum image sharpness. Another type of coating will help to shed raindrops on the optics lenses. Anyone who has been out observing wildlife and gets caught in the rain will appreciate the value that coating provides. All coatings are not created equal, and the bottom line is that coatings and the technology that goes into them play a significant role in just how razor sharp the image is. The real question is what is important to you? Do you want to see crisp detail or do you just want to see that the herd is moving north?


Magnification is the factor of how much closer the object in view is as compared to the actual distance away. For example, when viewing through a 7 power binoculars the object will appear 7x closer than it actually is. Mathematically speaking the magnification is the focal length of the eyepiece divided by the focal length of the objective lens. Optics can be either a fixed or variable magnification.  When the magnification is increased the field of view will always decrease. The most comfortable magnification range is 7 to 12 power. As magnification increases beyond that it begins to get harder to hold the optics still enough to avoid the movement in the view. At that point it is recommended that a tripod or other support be used to stabilize the optics or if your activity demands it, image stabilized binoculars will provide a great view when tripods are not conducive to the activity but when large magnifications are still needed.

Objective Lenses

The objective lens is the large lens that gathers the light waves coming from the object that you are viewing. Typically the size of the objective lens is one of the two component of the optics specification. For example, binoculars, rifle scopes and spotting scopes are specified by the magnification (range) and the objective size in millimeters.

Ocular Lenses

The ocular lens are the lenses that are looked through when using the optic- the eyepieces. The ocular lens is the primary lens that determines the width of view as well as the magnification.


Parallax is a displacement of an object when viewed from two different lines of sight. Parallax affects optical instruments such as binoculars and rifle scopes however it is most prevalent in rifle scopes, due to the fact that the position of the eye (eye relief) is farther away from the ocular lenses and the displacement is easier to ascertain. While looking through a rifle scope, parallax will cause the reticle to move off of the target when the user moves her head from side to side. Most scopes are designed to be parallax free at an predetermined distance, typically 100 yards for rifle scopes and 50 yards for shotgun or muzzleloader scopes. Parallax is typically not an issue when the scopes magnification is under 12 power. Above that magnification range scope manufacturers incorporate either a side parallax adjustment or an objective bell adjustment that will allow the user to set the approximate distance that he is shooting at and the scope will be parallax free at that setting.


Prisms are optical elements that refract light. Prisms can be made of different materials such as glass, plastic, or fluorite. Since the critical angle (see Physics of Optics below) for total internal reflection from a glass to air interface is less than 45 degrees, double glass (Porro) prisms with 45 and 90 degree angles are used to reflect light through 90 or 180 degree angles in many optical instruments such as binoculars and telescopes. The reflective ability of prisms is preferred over mirrors for several reasons. First, prisms reflect nearly 100% of the light whereas mirrors are not nearly as efficient. Secondly, mirrors tarnish over time where prisms do not. Finally a significant feature of prisms is that they will invert an image which counteracts the initial image inversion of the objective lens. There are several types of prisms including among others, the Porro prism previously mentioned, Porro-Abbe prism, as well as the Abbe-Koenig and Schmidt-Pechan, both roof prisms.


Commonly referred to as the cross hairs, reticles provide an aiming point that represents the point of impact of the projectile at some set distance away, typically 100 yards. The reticle is most  commonly represented as two thin intersecting lines that form a cross in the center of the view. Originally reticles were constructed out of hair or even spider webs. Later on thin wire was used which could be flattened to change the width. The most familiar type of reticle is the thin cross hairs, however they can be obscured by the background view during hunting. The duplex reticle is a combination of thick lines starting at the perimeter in each quadrant and then narrowing to thin lines which helps to assure accuracy and it won't obscure the target. The duplex reticle or one of its variations is the most commonly used reticle in America. Reticles are located on one of two focal planes in the scope. When located on the front focal plane, the reticle will grow in size along with the target when increasing magnification whereas reticles on the second focal plane remain a constant size. Most typically, magnification must be set to maximum setting when using a reticle that is located on the second focal plane. There are many alternate reticles available that assist with bullet drop compensation and ranging abilities. Additionally, reticles can be illuminated to assist viewing during low light situations such as at dawn or dusk. Ranging reticles and compasses are also available options from some binocular manufacturers.

Water/Fog Proofing

Quality optics will have desirable features such as sealed assemblies and quite often be internally pressurized with nitrogen or other inert gases to ward off moisture accumulation inside the optics. 

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All waves (water, vibration, sound, light) have common properties, the most important are their ability to transfer energy and information; however they cannot transfer matter. Waves are described and measured using quantities such as amplitude, speed, frequency, and wavelength. In this discussion we will focus our attention on light waves and how they are affected when traveling through sport optics such as binoculars, telescopes, spotting scopes, and rifle scopes. Waves behave differently when they pass from one medium to another, for example when viewing an object the incident light waves pass from air to the glass components of the optic.


Reflection and Transmission

What happens when an incident light wave reaches the end of one medium or abruptly changes from one medium to another? The incident wave separates- part is transmitted into the new medium (glass) and part is reflected back into the old medium (air). Trying to control or reduce the amount of light that is reflected is the reason that optic manufacturers have invested heavily into lens coatings. Their sole purpose is to reduce the reflection of light and increase the transmission rate of light into the optic. This will result in brighter, higher contrast images while at the same time reducing eye strain. Not all coatings are created equal. Optic manufacturers have defined very clear definitions about the extent of the coatings that are applied to the lens surface(s). Learn more about LENS COATINGS above in the glossary.

Reflection and Refraction

In the previous section we limited the discussion to waves that strike a boundary perpendicular to its surface. However most typically, the directions of the incident waves are not perpendicular to the boundary between the mediums. When the incident wave does not strike perpendicular, then the part of the wave that gets transmitted moves in a different direction from the incident wave; this transmitted wave is said to be refracted. Refraction of light waves is very important because it affects the sport optics ability to both focus and change the direction of light. A good example of refracted light is when you look at a pencil through a clear glass of water or at a fish in a pool of water. The fish or pencil is not where it appears to be.  As noted earlier, the amount of the light wave that gets reflected at the boundary is dependant upon how far an angle from perpendicular the incident wave strikes the boundary surface. The greater the incident angle, the greater the reflected angle. This leads to the Law of Reflection which states the angle of incidence is equal to the angle of reflection.

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Porro Prism

Porro Prism binoculars are considered the typical style of binoculars. Porro prism binoculars have a wider objective lens separation than the ocular lenses (eyepieces).  This offset design is due to the fact that porro prism binoculars have two right angle prisms in each barrel of the binoculars. The function of the prisms in any type of binoculars is to invert the image since the object in view becomes inverted (upside down) after the light waves pass through the objective lens. As a result porro prism binoculars are typically bulkier than roof prism binoculars. Porro prism designs do not require strict manufacturing tolerances, and as a result usually cost less than a comparable sized roof prism binoculars.

Roof Prism

Unlike porro prism binoculars, roof prism binoculars objective lens spacing lines up directly with the eyepiece spacing, making a much more streamlined and compact unit. The prisms overlap very closely which allows the streamlined design. This design literally comes at a price, since this style of binoculars are more difficult to manufacture they are also typically more expensive. 

Focusing Methods

There are two general methods for focusing binoculars. The most prominent method is the center focus where a center focus wheel is used to adjust both binocular barrels simultaneously. This style of focusing usually has a diopter adjustment that will accommodate differences between the eyes.

The second method adjusts each barrel separately. These binoculars typically have a very deep field of view and do not require refocusing when the objects location changes relative to the viewer's position. There are great benefits to this style of binoculars especially when viewing auto races, horse races and sporting events such as football games. 

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Compact binoculars typically have objective sizes in the 20 to 28 millimeter range. With small objective sizes the views through a compact pair of binoculars is much darker and the contrast is not nearly as defined when compared to mid or full sized binoculars. Many people associate compact binoculars with a low price. But that low price is gained at the compromise of many of the optical features of quality mid or full size optics that are truly desired. For activities such as birding or hunting the use of compact binoculars is not recommended by RCS Optics. Compacts do not have a good close focus specification; additionally, since they do not have good light gathering capabilities, their use a dawn or dusk is very limited. This fact is dictated by the exit pupil specification.

Mid Size

Mid sized binoculars have objective lens sizes from 30 millimeters up to 42 millimeters. This size range of binoculars is a good choice for many activities from birding to hunting to spectator sports. Some of the best features include the comfort handling and holding them in addition to their weight as they are typically lighter than the full size binoculars.  Mid sized binoculars are the most popular size.

Full Size

Full size binoculars have objective lens sizes from 42 millimeters up to about 56 millimeters. Full size binoculars have significant features that are often more desirable than what smaller binoculars can provide. Astronomy, marine, birding, and hunting are typical activities where the features of full sized binoculars are a great advantage. Full size binoculars have the greatest light gathering ability and coupled with higher magnification options provide the features needed in low light (dusk, dawn, and astronomy) applications as well as in applications such as marine use and birding. Their greatest disadvantage is their weight.

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Stadium Sports


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Rifle Scopes

Rifle scopes are scopes that are designed for use with a firearm. They typically will be fitted with a reticle (cross hair) and adjustment methods to insure that the point of aim aligns with the target point of impact at some specified distance down range. Rifle scopes can have either a fixed magnification or a variable magnification range. They are specified similarly to binoculars as the magnification and size of the objective lens are the two primary specifications that are cited.

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SPOTTING SCOPES Spotting scopes are a great tool for many outdoor enthusiasts including birders and hunters. Spotting scopes are basically small telescopes that are designed to observe nature rather than astronomical points of interest. Spotting scopes typically are fitted with very high magnification ranges when compared to binoculars. Typical magnification ranges extend from 15x to 60x or more. Often times different eyepieces are used to establish different magnification ranges. Spotting scope designs will have the eye piece (ocular lens) either in line with the objective or set at an angle to the objective lens.  A tripod is definitely recommended to ensure that the view through the optic will be steady and not shaking.

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