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All about Optics (most of this article)

A long range ballistics calculator

The Basics of Optics

For hundreds of years, people have used optics to enhance vision, as well as optimize effectiveness of shooting equipment. Whether glassing up that big buck, taking aim, or simply observing the natural world, great optics make great experiences. Optics can be very task-specific. For that reason, we want to make sure you’re armed with enough information to choose the right tool for the job. So come on in and let’s talk optics.

DETERMINING QUALITY OPTICAL GLASS

Quality optics use dense optical glass that is painstakingly designed, shaped, and polished to eliminate flaws. When a product features more sophisticated optical design techniques and glass, the results are better images. The quality of the optical glass will make a difference in how bright, sharp, and colorful a view will be.

  • Standard glass provides good image quality. 

  •  

    Extra-low Dispersion Glass achieves the highest possible image resolution, contrast, and color fidelity—exact properties vary among manufacturers. Some of the common names for this type of glass include: HD (High Density or High Definition), ED (Extra-low Dispersion) and XD (Extra-low Dispersion).

The lens blank manufacturing process is as follows: [2]

The glass batch ingredients for a desired glass type are mixed in a powder state, The powder mixture is melted together in a furnace, the fluid is further mixed while molten to maximize batch homogeneity, poured into lens blanks and annealed according to empirically determined time-temperature schedules. [2]

CONSTRUCTION OF OPTICS

You may pay more for products using higher quality materials, more sophisticated designs and stricter tolerances, but this adds up to greater reliability in the field.

  • Waterproof / Fogproof binoculars are sealed with o-rings to inhibit moisture, dust, and debris. The inside of the binocular is then purged of atmospheric air and filled with an inert gas that has no moisture content. This will prevent internal fogging from high humidity or altitude changes.
  • Nitrogen gas purging delivers fogproof, waterproof performance.
  • Argon gas purging guarantees superior fogproof and waterproof performance.

The simplest optical coatings are thin layers of metals, such as aluminium, which are deposited on glass substrates to make mirror surfaces, a process known as silvering. The metal used determines the reflection characteristics of the mirror; aluminium is the cheapest and most common coating, and yields a reflectivity of around 88%-92% over the visible spectrum. More expensive is silver, which has a reflectivity of 95%-99% even into the far infrared, but suffers from decreasing reflectivity (<90%) in the blue and ultraviolet spectral regions. Most expensive is gold, which gives excellent (98%-99%) reflectivity throughout the infrared, but limited reflectivity at wavelengths shorter than 550 nm, resulting in the typical gold colour. [3]

By controlling the thickness and density of metal coatings, it is possible to decrease the reflectivity and increase the transmission of the surface, resulting in a half-silvered mirror. These are sometimes used as "one-way mirrors" [3]

The other major type of optical coating is the dielectric coating (i.e. using materials with a different refractive index to the substrate). These are constructed from thin layers of materials such as magnesium fluoride, calcium fluoride, and various metal oxides, which are deposited onto the optical substrate. By careful choice of the exact composition, thickness, and number of these layers, it is possible to tailor the reflectivity and transmitivity of the coating to produce almost any desired characteristic. Reflection coefficients of surfaces can be reduced to less than 0.2%, producing an antireflection (AR) coating. Conversely, the reflectivity can be increased to greater than 99.99%, producing a high-reflector (HR) coating. The level of reflectivity can also be tuned to any particular value, for instance to produce a mirror that reflects 90% and transmits 10% of the light that falls on it, over some range of wavelengths. Such mirrors are often used as beamsplitters, and as output couplers in lasers. Alternatively, the coating can be designed such that the mirror reflects light only in a narrow band of wavelengths, producing an optical filter. [3]

The versatility of dielectric coatings leads to their use in many scientific optical instruments (such as lasers, optical microscopes, refracting telescopes, and interferometers) as well as consumer devices such as binoculars, spectacles, and photographic lenses. [3]

Dielectric layers are sometimes applied over top of metal films, either to provide a protective layer (as in silicon dioxide over aluminium), or to enhance the reflectivity of the metal film. Metal and dielectric combinations are also used to make advanced coatings that cannot be made any other way. One example is the so-called "perfect mirror", which exhibits high (but not perfect) reflection, with unusually low sensitivity to wavelength, angle, and polarization. [3]

Antireflection coatings [3]

Antireflection coatings are used to reduce reflection from surfaces. Whenever a ray of light moves from one medium to another (such as when light enters a sheet of glass after travelling through air), some portion of the light is reflected from the surface (known as the interface) between the two media. [3]

A number of different effects are used to reduce reflection. The simplest is to use a thin layer of material at the interface, with an index of refraction between those of the two media. The reflection is minimized when

n_{1}={\sqrt  {n_{0}n_{S}}},

where n_{1} is the index of the thin layer, and n_{0} and n_{S} are the indices of the two media. The optimum refractive indices for multiple coating layers at angles of incidence other than 0° is given by Moreno et al. (2005). [3]

Such coatings can reduce the reflection for ordinary glass from about 4% per surface to around 2%. These were the first type of antireflection coating known, having been discovered by Lord Rayleigh in 1886. He found that old, slightly tarnished pieces of glass transmitted more light than new, clean pieces due to this effect. [3]

Practical antireflection coatings rely on an intermediate layer not only for its direct reduction of reflection coefficient, but also use the interference effect of a thin layer. If the layer's thickness is controlled precisely such that it is exactly one-quarter of the wavelength of the light (a quarter-wave coating), the reflections from the front and back sides of the thin layer will destructively interfere and cancel each other. [3]

Interference in a quarter-wave antireflection coating [3]

In practice, the performance of a simple one-layer interference coating is limited by the fact that the reflections only exactly cancel for one wavelength of light at one angle, and by difficulties finding suitable materials. For ordinary glass (n≈1.5), the optimum coating index is n≈1.23. Few useful substances have the required refractive index. Magnesium fluoride (MgF2) is often used, since it is hard-wearing and can be easily applied to substrates using physical vapour deposition, even though its index is higher than desirable (n=1.38). With such coatings, reflection as low as 1% can be achieved on common glass, and better results can be obtained on higher index media. [3]

Further reduction is possible by using multiple coating layers, designed such that reflections from the surfaces undergo maximum destructive interference. By using two or more layers, broadband antireflection coatings which cover the visible range (400-700 nm) with maximum reflectivities of less than 0.5% are commonly achievable. Reflection in narrower wavelength bands can be as low as 0.1%. Alternatively, a series of layers with small differences in refractive index can be used to create a broadband antireflective coating by means of a refractive index gradient. [3]

High-reflection coatings [3]

 High-reflection (HR) coatings work the opposite way to antireflection coatings. The general idea is usually based on the periodic layer system composed from two materials, one with a high index, such aszinc sulfide(n=2.32) ortitanium dioxide(n=2.4) and low index material, such asmagnesium fluoride(n=1.38) orsilicon dioxide(n=1.49). This periodic system significantly enhances the reflectivity of the surface in the certain wavelength range calledband-stop, whose width is determined by the ratio of the two used indices only (for quarter-wave system), while the maximum reflectivity is increasing nearly up to 100% with a number of layers in thestack. The thicknesses of the layers are generally quarter-wave (then they yield to the broadest high reflection band in compare to the non-quarter-wave systems composed from the same materials), this time designed such that reflected beamsconstructivelyinterfere with one another to maximize reflection and minimize transmission. The best of these coatings built-up from deposited dielectric lossless materials on the perfect smooth surfaces can reach reflectivities greater than 99.999% (over a fairly narrow range of wavelengths). Common HR coatings can achieve 99.9% reflectivity over a broad wavelength range (tens of nanometers in the visible spectrum range). [3]

As for AR coatings, HR coatings are affected by the incidence angle of the light. When used away from normal incidence, the reflective range shifts to shorter wavelengths, and becomes polarization dependent. This effect can be exploited to produce coatings that polarize a light beam. [3]

By manipulating the exact thickness and composition of the layers in the reflective stack, the reflection characteristics can be tuned to a particular application, and may incorporate both high-reflective and anti-reflective wavelength regions. The coating can be designed as a long- or short-pass filter, a bandpass or notch filter, or a mirror with a specific reflectivity (useful in lasers). For example, the dichroic prism assembly used in some cameras requires two dielectric coatings, one long-wavelength pass filter reflecting light below 500 nm (to separate the blue component of the light), and one short-pass filter to reflect red light, above 600 nm wavelength. The remaining transmitted light is the green component. [3]

Extreme ultraviolet coatings [3]

In the EUV portion of the spectrum (wavelengths shorter than about 30 nm) nearly all materials absorb strongly, making it difficult to focus or otherwise manipulate light in this wavelength range. Telescopes such as TRACE or EIT that form images with EUV light use multilayer mirrors that are constructed of hundreds of alternating layers of a high-mass metal such as molybdenum or tungsten, and a low-mass spacer such as silicon, vacuum deposited onto a substrate such as glass. Each layer pair is designed to have a thickness equal to half the wavelength of light to be reflected. Constructive interference between scattered light from each layer causes the mirror to reflect EUV light of the desired wavelength as would a normal metal mirror in visible light. Using multilayer optics it is possible to reflect up to 70% of incident EUV light (at a particular wavelength chosen when the mirror is constructed). [3]

Transparent conductive coatings [3]

Transparent conductive coatings are used in applications where it is important that the coating conduct electricity or dissipate static charge. Conductive coatings are used to protect the aperture from electromagnetic Interference, while dissipative coatings are used to prevent the build-up of static electricity. Transparent conductive coatings are also used extensively to provide electrodes in situations where light is required to pass, for example in flat panel displaytechnologies and in many photoelectrochemical experiments. A common substance used in transparent conductive coatings is indium tin oxide (ITO). ITO is not very optically transparent, however. The layers must be thin to provide substantial transparency, particularly at the blue end of the spectrum. Using ITO, sheet resistances of 20 to 10,000 ohms per square can be achieved. An ITO coating may be combined with an antireflective coating to further improve transmittance. Other TCOs (Transparent Conductive Oxides) include AZO (Aluminium doped Zinc Oxide), which offers much better UV transmission than ITO. A special class of transparent conductive coatings applies to infrared films for theater-air military optics where IR transparent windows need to have (Radar) stealth (Stealth technology) properties. These are known as RAITs (Radar Attenuating / Infrared Transmitting) and include materials such as boron doped DLC (Diamond-like carbon) [3]

 

Tip 1

ANTI-REFLECTIVE LENS COATINGS

Metallic compounds, such as magnesium fluoride, are vaporized and applied to the optical glass in extremely thin layers to reduce internal reflections, light scattering and glare. The result of adding more layers of an anti-reflective lens coating to a greater number of glass surfaces is an improvement in image brightness, sharpness and contrast in low light.

Why anti-reflective coatings are needed. Anti-reflective coatings increase the amount of light that passes through the optical system so more light gets to your eye. The type and number of coatings applied to the lenses in a binocular or spotting scope make a significant difference in how brilliant and crisp the views will be.

Each time light strikes an uncoated glass surface about 4–5 percent of the light is reflected. Without lens coatings, almost 50 percent of the light could be lost as it passes through the multiple air-to-glass surfaces of a standard binocular or spotting scope.

 

Optical coatings

Tip 2

  • Levels of anti-reflective coatings
  • Fully multi-coated optics have all air-to-glass surfaces coated with multiple anti-reflective coating films, and offer the highest image quality.
  • Fully-coated optics have all air-to-glass surfaces coated with an anti-reflective coating film. Multi-coated optics have one or more surfaces coated with multiple anti-reflective coating films.
  • Coated optics have one or more surfaces coated with one or more anti-reflective coating films. 

SPECIFICATIONS

  • Knowing what features matter to your use of optics is important. What follows is an explanation of basic features and specifications to understand so you select optics that will perform to the level you need when out in the field.

EYE RELIEF

The term eye relief refers to the distance between the ocular lens and where the image comes to focus and the entire field of view can be viewed. Proper eye relief is important for safe, comfortable viewing.

  • Riflescopes: A minimum distance of three inches or more provides safe eye relief when viewing.
  • Binoculars and Spotting Scopes: Proper eye relief is important to people who must wear eyeglasses or sunglasses while looking through optics. However, anyone planning to view for long stretches of time will also benefit from optics with longer eye relief.

Binocular Eye CupsCLOSE FOCUS

This is the minimum distance to which you can focus an optic on your subject. Close focus is more important for some applications than others. For example, many binoculars will focus down to ten feet or less—a feature that is especially important for watching butterflies, insects and birds.

FIELD OF VIEW

Another important number to understand is the field of view. When looking through an optic, you’ll see the field of view as the area between the left and right edges of the image. The field of view can be measured either in linear feet or in angular degrees. (One degree equals 52.5 feet).

  • Riflescopes: measured in feet @ 100 yards
  • Binoculars: measured in feet @ 1,000 yards
  • Spotting Scopes: measured in feet @ 1,000 yards

Optics Field of ViewEXIT PUPIL

This is the beam of light that exits each eyepiece and enters the user’s eyes. You’ll want to have an exit pupil that is adequate for the lighting situation in which you’ll be viewing. A person’s eye pupil can dilate from roughly 2 mm to 8 mm, depending on the person’s age and the lighting situation:

  • In bright light the pupil will dilate to about 2–3 mm.
  • At dawn or dusk the pupil will dilate to about 4–5 mm.
  • In dark light the pupil will dilate to about 7–8 mm.

A larger exit pupil will deliver brighter images—especially under low light conditions.

Exit Pupil

Tip 3

RESOLUTION

Resolution refers to the ability of an optic to distinguish details. A resolution chart contains groups of lines set in a series with progressively smaller spacing—a design used to ascertain the limiting number of lines per millimeter that optics can resolve.

Optics Resolution Chart

WARRANTY

A manufacturer’s warranty ought to be considered a feature of the binocular—especially if you use the optics outdoors where anything can happen. Most warranties offer a warranty limited only to initial defects with no protection from accidental damage or regular wear and tear. Progressive warranties cover optics in any situation, no matter what happens or who is at fault.

 

TRADE-OFFS TO CONSIDER

Yes, there are trade-offs and, no, there are no perfect optics. So, consider the following trade-offs when selecting optics. 

  • OBJECTIVE LENS SIZE - Objective lens size is the main trade-off to consider. A larger objective lens will deliver brighter images, especially under low light conditions, but it will be heavier and bulkier than a smaller lens. Think about how much you want to carry!
  • OPTICAL GLASS QUALITY - Optical glass changes in weight as the quality increases. Vortex offsets the extra weight of the high-quality glass components by using rugged, yet lightweight, housing materials.
  • MAGNIFICATION - Choosing the higher magnification option has benefits, but it may not always be the best choice for observation. 
    Binoculars: As the magnification increases, you’ll see a shallower depth of field, a diminished field of view, and you may experience a greater chance of image shake when viewing. Spotting Scopes: As the magnification increases, you’ll see a reduction in image brightness.
  • CLOSE FOCUS AND DEPTH OF FIELD - In general, optics with a close focus will generally have a shallow depth of field.

MORE OPTICS TERMS

  • Alignment or Collimation - All elements (lenses or prisms) are in line along the optical axis. The misalignment of elements results in diminished performance and can cause eye strain and fatigue. 
  • Astigmatism - Because the lenses in a binocular or spotting scope usually have a curved shape, the light rays passing through the lens will not all converge on the same focal plane. If this physical reality isn’t remedied in the overall optical design, images will either be in focus in the center area or at the edge—but not in both areas at the same time. Astigmatism cannot be eliminated completely, but it can be kept to a minimum. Avoid optics that exhibit too much astigmatism.
  • Chromatic Aberrations - Diminished resolution and color fidelity display as green or purple fringing. This is the result of a physical reality of color. Different colors move at slightly different wavelengths and will have slightly different focal lengths when passing through optical glass. The XD and ED glass types reduce or eliminate this inherent problem of chromatic aberrations. 
  • Contrast - This refers to differences in brightness between the light and dark areas of an image. Because we see much of the color spectrum, contrast also refers to differences in the dimensions of hue, saturation, brightness, or lightness. Optics with superior contrast transmit colors that appear very dense and well-saturated.
  • Distortion - This is the inability of an optical system to deliver an image that is a true-to-scale reproduction of an object. There are two types of distortion. In either case, the distortion is due to a poor or compromised optical design. Any binocular or scope that exhibits distortion should be avoided.
  • Barrel distortion - Image bows outward and looks bulged.
  • Pincushion distortion - Image bends inward.
  • Light Transmission - This is the percentage of light that passes through the binocular, spotting scope, or riflescope to reach the user’s eyes. Light transmission will be higher through more expensive optics than through modestly priced optics due to better optical designs, glass quality, and improved optical coatings.
  • Resolution - Essentially the same as image sharpness, resolution is the ability of the binocular to separate and distinguish thin lines with clarity.

 

Riflescopes

Ready. Aim. Fire!

  • Riflescopes and their features are as varied as the firearms they can sit atop.
  • The firearm, as well as its intended application will dictate which riflescope will be the best fit. Understanding the basics will make the right choice clear.

UNDERSTANDING THE CONTROLS

Windage, oculars and parallax—oh my! Riflescopes generally have several adjustable features. When broken down to the basics, many are commonly shared and relatively simple. Once basic feature terminology and their functions are understood, you’ll be able to select the right riflescope with pinpoint accuracy.

Scope Controls

TUBE DIAMETER

Riflescope main tubes come in several diameters, including 1 inch, 30 mm, 34 mm and 35 mm. Larger diameter tubes can provide increased travel ranges for windage and elevation adjustments as well as greater strengths. Being aware of tube diameter is also very important when selecting rings to mount the scope.

Riflescope Tube sizes

Tip 6

OCULAR FOCUS

Use the ocular focus to tune the reticle image for maximum sharpness. This adjustment will be slightly different for every shooter, and only needs to be set one time. To adjust, begin by backing the focus out until the reticle is clearly fuzzy. While taking short, quick looks through the scope, turn the focus in until reticle image is sharp and crisp to the eye immediately upon viewing. Do NOT use this focus to adjust the target image.

 

Scope Reticle Adjustment

MAGNIFICATION ADJUSTMENT

Use the magnification adjustment to change the “power” level of the riflescope— adjusting from low to high magnification depending on the shooter’s preference.

  • Lower magnifications will provide brighter images and wider fields of view which can be helpful in low light and/or closerange shooting and with moving targets.
  • Higher magnifications will have narrower fields of view and dimmer images, but will offer better ability to shoot smaller targets at longer ranges.

Scope Magnification Adjustment

 

ELEVATION AND WINDAGE TURRETS

Turrets are used to adjust the bullet’s point of impact down range, and will be marked in either MOA or MRAD scales. Turrets come in several styles, depending on user preferences.

  • Exposed target-style turrets are used by long range shooters who routinely “dial” elevation corrections for bullet drop at long range.

Riflescope Windage and Elevation

  • Capped style turrets are often used by shorter range shooters and hunters, who may prefer the security and lower profile of this type.

Riflescope Capped Turrets

 

Arc Measurements

MRAD (Milliradian) arc measurements are based on the concept of the radian. A radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle. There are 6.283 radians in all circles. Since there are 1,000 milliradians in a radian, there are 6,283 milliradians (MRADs) in a circle. An MRAD will always subtend 3.6 inches for each 100 yards distance.

Most riflescopes using MRAD turrets will use 1/10 mrad mechanical clicks which subtend .36 inches for each 100 yards of distance.

Rifle Scope MRAD Bullet Drop

MOA (Minute of Angle) arc measurements are based on the concept of degrees and minutes in a circle. There are 360 degrees in a circle, 60 minutes in a degree for a total of 21,600 minutes in a circle. An MOA will always subtend 1.05 inches for each 100 yards distance. Most riflescopes using MOA turrets will use ¼ minute mechanical “clicks” on the turret which subtend .26 inches for each 100 yards distance.


Bullet Drop in Degrees

IMAGE SHARPNESS

Some riflescope models feature an adjustment that allows you to tune the target image for maximum sharpness. This adjustment may be on the objective lens or near the turrets on the side of the riflescope.

Adjustable Objective Lens Focus – This adjustment dial is marked with approximate yardages to aid in initial setting, and should be matched to the targets distance. Final focus setting should be checked by moving shooters head back and forth slightly, watching for any shift of the reticle on the target (parallax). If shift is observed, the dial should be adjusted slightly until shift is removed. Once this focus is correctly set, shooting errors due to parallax will be eliminated.

Rifle Scope Objective Focus 


Side Focus Adjustment – This adjustment serves the exact same purpose as an adjustable objective, but is more conveniently located on the left side of the riflescope. The adjustment dial is marked with approximate yardages to aid in initial setting, and should be matched to the targets distance. Final focus setting should be checked by moving shooters head back and forth slightly, watching for any shift of the reticle on the target (parallax). If shift is observed, the dial should be adjusted slightly until shift is removed. Once this focus is correctly set, shooting errors due to parallax will be eliminated.

Rifle Scope Side Lens Focus

What is parallax?

Parallax is a phenomenon that results when the target image does not quite fall on the same optical plane as the reticle within the scope. This can cause an apparent movement of the reticle in relation to the target if the shooter’s eye is off-centered. 

Parallax Eye Not Centered

Parallax Eye Centered

Parallax Eye Focused

RETICLE ILLUMINATION ADJUSTMENT

Use the reticle illumination adjustment to “light up” all or a portion of the reticle within a riflescope—allowing the reticle to be more easily seen against a dark background. The intensity level can usually be adjusted and is commonly placed on the ocular or left side of the scope, though it can be located in other positions. Illumination is normally powered by a small watch type battery.

Scope Illumination Adjustment

 

ZERO STOP ADJUSTMENT

Use the zero stop adjustment to prevent the elevation turret from being rotated downward past the point of original zero. It is most useful for shooters who routinely adjust the elevation turret “up” for long range shots, allowing them to always easily and accurately return “down” to their original zero setting. Zero stops are usually seen on higher quality long range or tactical riflescopes.

Scope zero adjustment

UNDERSTANDING THE NUMBERS

THE RIFLESCOPE CONFIGURATION

Magnification is indicated by the first set of numbers in the example of a 4–16x50 riflescope—the magnification ranges from 4x up to 16x. Some riflescopes do not have a zoom eyepiece and use a single number to indicate a fixed magnification, as in a 2x20 scope.

  • Magnification is indicated by the first set of numbers in the example of a 4–16x50 riflescope—the magnification ranges from 4x up to 16x. Some riflescopes do not have a zoom eyepiece and use a single number to indicate a fixed magnification, as in a 2x20 scope.

 Scope Magnification Scale

  • Objective Lens Size determines how much light can be gathered to form an image. [4] It is usually expressed in millimeters.is indicated by the last number in the 4–16x50 example—referring to the diameter of the objective lens in millimeters. If all other things are equal, larger objectives can yield brighter images at high magnifications. This is an advantage for hunting at dusk and dawn when animals are most active.

 Scope Objective Lens

EYE RELIEF

With proper eye relief, there will be a space cushion that protects the eye from recoil of the firearm. Keep in mind that eye relief typically decreases as magnification increases.

Scope Eye Releif

 

UNDERSTANDING RETICLES

From the simple Plex crosshair to first focal plane hashmarkbased, mrad reticles with wind dot references—every reticle shines under certain conditions and when paired with an appropriate firearm.

FIRST AND SECOND FOCAL PLANE RETICLES

All reticles will be termed either first (FFP) or second (SFP) focal plane, depending on their internal location within the riflescope.

FFP – This style of reticle will grow and shrink as magnification is changed. The main advantage to this style reticle is that the reticle subtensions used for ranging, bullet drop compensation and wind drift corrections are always accurate at any magnification.

FFP Reticle

SFP – This style of reticle does not change size when magnification is changed. The advantage to this style of reticle is that it always maintains the same ideal visual appearance and will not appear “too fine” at low magnification or “too heavy” at high magnifications.

SFP Reticle

HOW TO RANGE WITH MRAD AND MOA RETICLES

Using simple formulas, both MOA and mrad hashmarked reticles can be used to estimate distance. This is a useful skill—and provides a good back-up should your laser rangefinder fail or lose battery power.

To range with a reticle formula, you can use either the vertical or horizontal scale. Place the reticle on a target of known width or height and read the number of mrads or MOAs spanned. You will obtain maximum accuracy in ranging by calculating as exact a measurement as possible—down to fractions of an mrad or MOA. Accurate measuring will depend on a very steady hold. The rifle should be solidly braced using a rest, bipod or sling when measuring the size of the target or nearby object. Once you have an accurate reading, use a formula to calculate the distance.

MRAD Ranging Formulas

 

tip 7

Red Dots [5]

A red dot sight is a common classification for a type of non-magnifying reflector (or reflex) sight for firearms, and other devices that require aiming, that gives the user an aimpoint in the form of an illuminated red dot. A standard design uses a red light-emitting diode (LED) at the focus of collimating optics which generates a dot style illuminated reticle that stays in alignment with the weapon the sight is attached to regardless of eye position (nearly parallax free). They are considered to be fast acquisition and easy to use gun sights for target shooting, hunting, and in police and military applications. Aside from firearm applications, they are also used on cameras and telescopes. On cameras they are used to photograph flying aircraft, birds in flight, and other distant, quickly moving subjects. Telescopes have a narrow field of view and therefore are often equipped with a secondary "finder scope" such as a red dot sight.[5]

Red Dot

The typical configuration for a red dot sight is a tilted spherical mirror reflector with a red light-emitting diode (LED) at its off axis focus. The mirror has a partially silvered multilayer dielectric dichroic coating designed to reflect just the red spectrum allowing it to pass through most other light. The LED used is usually deep red 670 nanometre wavelength since they are very bright, are high contrast against a green scene, and work well with a dichroic coating since they are near one end of the visible spectrum. The size of the dot generated by the LED is controlled by an aperture hole in front of it made from metal or coated glass. The LED as a reticle is an innovation that greatly improves the reliability and general usefulness of the sight. There is no need for other optical elements to focus light behind a reticle. And the LED itself is solid state and consumes very little power, allowing battery powered sights to run for hundreds and even tens of thousands of hours. Using a "dot" shaped reticle also greatly simplifies the sight since the small diameter image does not require a sophisticated optical reflector to focus it. More complex reticle patterns such as cross hairs or concentric circles can be used but need more complex aberration free optics. [5]

Like other reflector sights, the collimated image of the red dot is only truly parallax free at infinity, with an error circle equal to the diameter of the collimating optics for any target at a finite distance. This is compensated for by keeping the dot in the middle of the optical window (sighting down the sight's optical axis) Some manufacturers modify the focus of the LED/optical collimator combination, making models with the optical collimator set to focus the dot at a finite distance. These have a maximum amount of parallax due to eye movement, equal to the size of the optical window, at close range, diminishing to a minimal amount at the set distance (somewhere around a desired target range of 25–50 yards) [5]

Sights may also use a more sophisticated optical system that compensates for off axis spherical aberration, an error that can cause the dot position to diverge off the sight's optical axis with change in eye position. The optics used is a type of mangin mirror system, consisting of a meniscus lens corrector element combined with the semi-reflective mirror, sometimes referred to in advertising as a "two lens" or "double lens" system. Although these are referred to as "parallax free" sights, the system only keeps the aiming dot in alignment with the sight itself and does not compensate the inherent parallax errors induced by a collimated sight. [5]

Red dot sights generally fall into two categories, "tube" or "open" designs. "Tube sights" look similar to a standard telescopic sight, with a cylindrical tube containing the optics. Tube sights offer the option of fitted dust covers and the ability to add filters, such as polarizing or haze filters, and glare reducing sunshades. Since a reflector sight only really needs a single optical surface, the "reflector", the tube is not needed. This allows for non-tubed "open sights" that consist of a flat base, with a single loop of material to support the reflective optics. [5]

Most red dot sights have either active or passive adjustments for the dot brightness, allowing a very bright dot for high visibility in bright conditions, and a very dim dot to prevent loss of night vision in low light conditions. [5]

Vortex Red DotRed dot sight reticles are measured in minutes of angle, or "MOA". MOA is a convenient measure for shooters using English units, since 1 MOA subtends approximately 1.0472 inches at a distance of 100 yards (91.44 m). This is generally rounded to 1 inch at 100 yards, which makes MOA a handy unit to use in ballistics. One of the most common reticles used in red dot sights is a small dot, covering 5 MOA (1.5 mrad). The 5 MOA (1.5 mrad) dot is small enough not to obscure most targets, and large enough to quickly acquire a proper "sight picture". For many types of action shooting, a larger dot is preferred; 7 (2.0 mrad), 10 (2.9 mrad), 15 (4.4 mrad) or even 20 MOA (5.8 mrad) dots or rings are used; often these will be combined with horizontal and/or vertical lines to provide a level reference. [5]

 

Binoculars

What binocular should I get? The answer to this question is generally found by asking another. What do you plan on using it for? A person scouring a vast western landscape will have different needs from another who fi nds themself immersed in a stand of Midwest hardwoods. Read ahead about the various features of different binoculars and you’ll SEE what we’re talking about.

BINOCULAR DESIGN

There are three main binocular designs: the roof prism, Porro prism, and reverse Porro prism. These designs come in a variety of weights and sizes. The greatest factor in determining the weight of a binocular is the size of the objective lens: the larger the lens, the heavier the binocular.

  • Compact binoculars generally have objective lenses of 28 mm or less and can weigh from a few ounces to under a pound.
  • Mid-size binoculars include models with objective lenses between 30 mm and 35 mm.
  • Full-size binoculars generally have objective lenses over 35 mm and can weigh from twenty ounces to around two pounds.

Binocular Models

ROOF PRISM

Named for the roof-like appearance of the prisms, the roof prism binocular has objective lenses and eyepieces positioned in a straight line and is appreciated for a streamlined, durable chassis. Phase correction coatings on the prism glass keeps the light in correct color phases—enhancing the resolution, contrast and color fidelity. Fine quality in this complex prism design is possible as a result of care in engineering and design.

Binoculars_roof_prism

PORRO PRISM

Many people will recognize the traditional binocular shape of a Porro prism by its offset barrels. Named after the Italian optical designer, Ignazio Porro, Porro prism binoculars have objective lenses that are spaced wider apart than the eyepieces. This design offers a rich depth of field, wide field of view, a three-dimensional image, and delivers good quality at a reasonable cost.

Binoculars Porro Prism

REVERSE PORRO PRISM

The reverse Porro prism is a compact version of the full-size Porro prism binocular with the eyepieces spaced wider apart than the objective lenses.

Binoculars Reverse Porro Prism

THE NUMBERS

IDENTIFYING THE CONFIGURATION

When you look at your binocular, you’ll notice numbers like 10x50 (read as “ten by fifty”) printed on the binocular.

 

The first number (10x) refers to the magnification provided by the binocular (or how many times larger an object will appear than when viewed without magnification). Binoculars vary in magnification, but 8x and 10x are most common.

tip 8

The second number (50) refers to the diameter of the objective lens in millimeters. Objective lenses vary in size from 15 mm to 50 mm and beyond. The size of the objective lens determines how much light the binoculars can receive and how bright the resulting images will be. The size of the objective lens also affects the size of a binocular.

  • Exit pupil is especially important for viewing in low light conditions. If your primary time for viewing is during the bright light of day, then a binocular with a smaller objective lens of 26 mm or less will do just fine. If you want the brightest possible image during near-dark conditions, you’ll want to choose a binocular with an objective lens in the 33 mm to 56 mm range.
  • Wide field of view has advantages when following fast-moving action and scanning dense habitats. The field of view is measured in feet at 1,000 yards or degrees: Example: 388 feet @ 1000 yards 6.0 degrees
  • Close-focus binocular will focus down to ten feet or less. This feature is especially important for watching birds, insects and butterflies.

BASIC ADJUSTMENTS

ADJUST THE INTERPUPILLARY DISTANCE

The interpupillary distance (IPD) is a measurement of the distance between the centers of a person’s left and right eye pupils. A binocular also has an IPD measurement that can be adjusted.

The hinged design of a binocular allows you to match the IPD of your eyes to that of the binocular so that you see a single image that is free of shading. If the IPD is not correctly adjusted, you may see shading over part of the image. With correctly adjusted binoculars, you will see a single image without the shading.

Binocular IPD

To adjust the IPD of your binocular, simply rotate the binocular barrels inward or outward to line up the ocular lenses with your eyes.

Binocular IPD Adjustment

ADJUST THE EYECUPS

Adjusting the eyecups up or down allows the user to see a full field of view. This is important for people who must use eyeglasses or sunglasses. The two main styles of eyecup design are:

  • Retractable eyecups that twist up and down. Multi-position eyecups let you choose the most comfortable position.
  • Flexible eyecups that fold back for maximum eye relief with eyeglasses.

 Binocular Eyecups

  • With Glasses – If you wear eyeglasses or sunglasses, rest the eyecups of the binocular against your glasses with the eyecups folded back or twisted down. If the eyecups stay fully extended when wearing eyeglasses, images will appear as if you are looking at them through a tunnel.
  • Without Glasses – If you do not wear eyeglasses or sunglasses, extend the eyecups to provide the proper distance for seeing the full field of view. If the eyecups do not stay fully extended, you may see black crescents in the field of view.

Binocular Eyecup Adjustment

PROPERLY FOCUS THE BINOCULAR

For the best views, follow this two-step process to properly adjust the center focus and diopter. Choose an object that is about 20 yards away from you and stay in the same spot until you have adjusted the binocular for your eyes.

1. Adjust the center focus – Start by closing your right eye or covering the right objective lens with your hand. Focus your left eye on the object and adjust the center focus wheel until the image is in focus. Leave the center focus in this position as you adjust the diopter.

Binocular Center Focus Adjustment

2. Adjust the diopter – Start by closing your left eye or covering the left objective lens with your hand. Look through your right eye and adjust the diopter ring (generally found on the right eyepiece) until the object is in focus. Make note of this diopter setting in case you need to set it again. From this point on, you will only need to use the center focus wheel.

Binocular Diopter Adjustment

SPOTTING SCOPES

When true long-distance spotting and subject evaluation are the name of the game, it’s time to break out a spotting scope. As with other optics, spotting scopes have specifi c features you’ll want to be familiar with. Zoom in on the facts to ensure all your spotting needs are met.

SPOTTING SCOPE DESIGN

Spotting scopes provide higher magnification than available through most binoculars and are designed for viewing wildlife and landscapes at longer distances. In many cases, manufacturers make a spotting scope design available with both an angled and a straight body style. Though one design is not better than the other, each offers distinct advantages.

  • The angled body features an eyepiece that is set at a 45-degree angle. This style lets people of different heights share without adjusting the tripod. Because angled scopes can sit lower on a tripod, users will benefit from the added stability.

Spotting Scope Design

  • The straight body features an eyepiece in line with the objective lens. This natural line of sight works well with a car window mount.

Spotting Scope

THE NUMBERS

IDENTIFYING THE CONFIGURATION

The name of a spotting scope frequently includes a group of numbers such as 20–60x85. This range of numbers is called the configuration and indicates the magnification and the size of the objective lens.


The first set of numbers (20–60x) indicates the magnification range. Since spotting scopes feature high magnifications for longdistance viewing and large objective lenses, these optics must be mounted on a tripod.

Tip 10

The last number (85) indicates the size of the objective lens in millimeters. This size directly affects the overall size of the spotting scope resulting in anything from extremely compact to full-size models.

Spotting Scope Sizes

Tip 11

BASIC ADJUSTMENTS

ADJUST THE EYECUP

Spotting scopes typically feature an adjustable eyecup in one of two styles: twist or fold. Adjusting the eyecup up or down allows you to see a full field of view whether or not you wear eyeglasses. Even if you wear sunglasses, making this adjustment will enhance your viewing experience.

Spotting Scope Eyecup Adjustment

ADJUST THE MAGNIFICATION

Change the magnification of your spotting scope by simply turning the magnification adjustment ring in a clockwise or counterclockwise direction.

Spotting Scope Magnification Adjustment

ADJUST THE FOCUS

Some spotting scopes provide both fast and fine focus dials. After setting the magnification, some refocusing is usually required.

1. Slowly turn the fast focus dial until the subject is nearly in focus.
2. Turn the fine focus dial to pick out the finest details.

Spotting Scope Focus Adjustment

ADJUST THE VIEWING ANGLE

Some spotting scopes provide a rotating tripod collar that allows you to rotate the spotting scope body for greater viewing flexibility.

Spotting Scope Viewing Angle Adjustment

ADJUST THE SUNSHADE

Some spotting scopes provide a built-in sunshade that extends to effectively block out disturbing stray light. The sunshade also shields the objective lens from mechanical damage and guards against soiling by fingerprints and precipitation

Spotting Scope Sunshade Adjustment

References

[1] Vortex Canada https://www.vortexcanada.net/

[2] https://en.wikipedia.org/wiki/Optical_lens_design

[3] https://en.wikipedia.org/wiki/Optical_coating

[4] https://en.wikipedia.org/wiki/Telescopic_sight

[5] https://en.wikipedia.org/wiki/Red_dot_sight

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