ASTYGMATISM

Dave Brown
May 09, 2016
By Dave Brown
As people age, the shape of their eye's cornea and lens may tend to become more oblong and less perfectly spherical. This is known as astigmatism, and will cause the eye to focus on multiple points on the back of the eyeball instead of on one pinpoint spot. This is an issue that will become more common as more agencies adopt patrol carbines for street officers. It will particularly impact older officers whose eyesight may not be as sharp as it was when they were 20.

Red dot sights may be one of the greatest advancements in firearms accuracy for short or intermediate-range target shooting since someone cut a notch into the top of the hammer on their single-action revolver. Ideal for the modern patrol carbine, red dot sights are lightweight, quick to acquire and extremely accurate within their design range.

They also have the huge advantage of allowing an officer to keep both eyes open and focus on the threat instead of the front sight.

For cross-dominant officers, it also doesn't matter which eye sees the dot or where the electronic dot is aligned in the tube; where it appears is where the round goes.

Their singular pinpoint dot has one drawback however, and this is an issue that will become more common as more agencies adopt patrol carbines for street officers. It will particularly impact older officers whose eyesight may not be as sharp as it was when they were 20.

As people age, the shape of their eye's cornea and lens may tend to become more oblong and less perfectly spherical. This is known as astigmatism, and will cause the eye to focus on multiple points on the back of the eyeball instead of on one pinpoint spot.

{Astigmatism and accuracy}

Everyone has some degree of astigmatism. No eyeball is perfectly symmetrical. There are degrees, much like there are degrees in all correctable vision problems.

Astigmatism is sometimes first noticed as just a faint uncomfortable feeling when driving at night; distant lights seem more smears than points. It often depends on how much one is willing to put up with this slight distortion and so can go undetected and thus uncorrected for many years – but as soon as that officer tries using a modern red dot sight, they may see multiple or smeared dots or, in the worst case, a big faded globe of light instead of one sharp dot. Light sourced optics such as AimPoint, Trijicon and EOTech are particularly prone to this multiple dot characteristic.

Astigmatism rarely affects both eyes equally. Some report multiple dots only in one eye and a clearly defined center point in the other. Another problem is that the multiple dots may come and go, depending on everything from how much sleep the officer had to how much coffee they drank that day. What appears as three dots one day may suddenly look like a bunch of grapes hanging from a tree the next.

The result is no clearly defined center dot and difficulty discerning which dot to consistently use to aim for pinpoint accuracy.

{Astigmatism and red dot sights}

A red dot sight is simply a non-magnifying optical sight that superimposes a glowing "dot" or lighted reticle over a weapon's impact point. Powered by batteries, the brightness can be adjusted for various lighting conditions, including even the invisible part of the spectrum for use with night vision on certain models.

The exact position of the dot can be adjusted up and down or left and right, much like a telescopic sight, to match the exact point of impact at a chosen distance. The dot or reticle itself is focused at or near infinity, allowing a unique ability to focus on a target instead of the sight itself.

There is no magnification in the optics so there is no requirement for a specific placement of the eyeball to the rear element, a dimension known as eye relief distance. This means that the red dot sight can be placed nearly anywhere on a rail and still work effectively.

In actual use, an officer keeps both eyes open and focused on the threat. A lighted impact point is continuously superimposed over the image, and with both eyes open and the focus now way out in front of the weapon, the sight body becomes almost non-existent.

The dot or reticle may be generated by a laser or light-emitting diode (LED) and can consist of a single dot, such as the Aimpoint design, or multiple dots forming a targeting reticle, such as the EOTech.

Many red dots are known as "reflex sights" and project the dot into the user's sight using a partially-reflective parabolic mirror. Others, known as "holographic sights," use a laser pattern to project a more elaborate reticle image by hologram inside the glass of a prism.

Most electronic red dot sights are termed "parallax-free" near the center of the scope and some are considered virtually parallax-free at any viewing angle. This means that, unlike a magnifying scope, the dot or reticle does not need to be in the center of the tube or viewing glass; it will always indicate the exact impact point at the distance it is sighted for, no matter where the eye aligns with the optic.

Battery life is dependant on design. The LED emitters on the combat-tested Aimpoint sight can be left on for years. Once equally popular in combat, holographic sights such as the EOTech, because of elaborate reticles made up of multiple dots, have much shorter battery lives but are often designed to shut themselves off if not used after a certain period of hours.

Just as nearly everyone has some degree of astigmatism, all red dot sights suffer from some degree of smeared or multiple dots that will vary widely from user to user. While some report that a reflex design is less prone to this phenomenon, tests by have not supported this. Everyone is different and what they see will be different. Even with the same user, one eye can see multiple dots while the other clearly sees one.

{Is this a problem?}

In practical terms, some degree of astigmatism is unlikely to make much difference to most officers at the typical encounter distances of patrol carbines.

One could argue that, once a red dot sight is properly sighted in at the chosen distance, multiple dots or a partially smeared image will not affect accuracy to any practical degree. This is quite true. In real life encounters, one rarely gets to fire a single aimed shot from a well-braced position. In fact, red dot sights are not made for those single aimed shots at a distance anyway; they are designed for close, fast and accurate shots, usually within 100 meters or less.

This is why astigmatism may not be a major concern for many officers. Other than the fact that they will see multiple dots, they simply put the impact point onto the target and, if necessary, pull the trigger. Red dot sights are most suited for the close and intermediate-range dynamic encounter, after all.

Sighting in will be more problematic in firing careful shots from a very stabilized rest in order to ensure maximum accuracy. This is where officers with a greater degree of astigmatism may need others to sight in their rifle for them.

{A fix}

The good news is that the problem can be corrected. The bad news is that this correction may require eyeglasses, contacts or laser surgery.

With proper vision testing from an optometrist or ophthalmologist and good prescription lenses, most astigmatism can be taken care of. Complications arise with lens corrections as officers approach 40 years of age and begin suffering from natural near-sightedness as well as astigmatism.

When consulting with an eyesight professional, it is most helpful to take along the red dot sight and test or discuss the various options. Astigmatism can also be corrected by certain types of contact lenses. Consult with your eye doctor to determine if contacts are a good option.

{Alternate optic solutions}

If astigmatism is becoming frustrating and the officer can not or chooses not to correct it with contacts, eyeglasses or laser surgery, there are alternate optical solutions for short and intermediate range carbines or rifles.

Low magnification or variable power scopes that go down to or near one power magnification (1X) can be a good choice. The best ones for officers with astigmatism are those with etched reticles. Having the crosshair etched directly on an internal glass element instead of stringing thin internal wires inside the scope tube seems to help many shooters with mild astigmatism. Etched designs can also come with illuminated reticles for low light conditions.

Etched reticle scopes are much less prone to loss of zero if dropped and the more expensive models can be made virtually unbreakable. Most come with a variable diopter ring at the rear. While intended to help correct for near or far focus issue, it can also be helpful with some degrees of astigmatism.

Some users have reported that flipping up the rear backup sights to surround the red dot with the iron sight's rear aperture helps reduce multiple dot issues. This somewhat forces the eye to reshape and focus more rearward.

Prismatic or prism scopes use a glass prism for magnification instead of internal lenses. They can be almost as compact and robust as a combat-tested red dot sight and many come with etched reticles and illuminated reticle patterns. They also use focusing eyepieces to correct for focus differences and can be useful in helping correct some degrees of astigmatism.

The downside to any scope is the fact that you lose the fast acquisition and unlimited eye relief of the red dot sight. The eyeball must be placed a precise distance away from the rear lens to work properly. They also have some degree of parallax error, where the actual impact point may vary slightly if the crosshairs or reticles are not centered in the scope tube. They are also much bulkier than a red dot sight such as the highly-recommended Aimpoint.

When properly mounted and fitted to the shooter, a low or one-power prism scope such as the Burris AR-332 – or variable power scope that goes down to near one power, such as the Burris XTR II 1.5-8X scope (as featured on the cover) – can give nearly the same speed as a red dot sight and with fewer astigmatism issues. Etched reticles also have another advantage; if the batteries die or the illuminated reticle fails, the sight can still be used in most lighting conditions using the reticle etched on the internal glass.

{Special note for EOTech users}

L-3 Communications (the parent company of EOTech) reached a $25 million settlement in 2015 with the US government over problems with EOTech optics. Once widely used in police and military operations, some of the company's claims about their sights have proven to be false or misleading. For example, EOTech claimed that its holographic sights were parallax free at any distance and could be used in operational temperatures from -40 to +60 degrees Celsius.

Actual testing showed massive zero shifts of up to 12 minutes-of-angle (MOA) at 0 degrees C, and up to 20 MOA at -15 degrees C. Parallax error was detected as temperatures approach 0, and many samples were unable to return to the same zero as temperatures increased.

Additionally, humidity could dim reticles in older optics. It was alleged that EOTech concealed this information from end users and the government long after it was discovered, continued to maintain that existing optics met military specifications and concealed information about failures in its holographic sight system performance from government contract bidders and testing facilities.

EOTech is currently offering refunds to users. As of December 2016, no solution has been found for the drifting zero with temperature changes. In the long-term test sample EOTech provided to the reticle has severely dimmed over the test period. When asked for a solution, L3 Communications simply suggested buying a new EOTech.

A 12 MOA shift at 0 degrees means the sight can be as much as 12 inches off at 100 yards. This is unacceptable for a police weapons sight system, especially in Canada where temperatures can routinely dip far below 0. no longer recommends EOTech for duty use.

{Best solution}

There is no one best solution for every situation. Some officers find low power prism scopes to be an excellent choice. Others have success by flipping up the rear iron backup sight or can simply live with the multiple dots of a red dot sight. They don't want to lose the fast acquisition, lightweight design and long battery life of a combat-tested electronic sight.

While researching this article over the course of three years, has tested nearly every modern patrol rifle optic on the market. We remain fans of the consistently reliable and virtually unbreakable Aimpoint, even with fairly pronounced astigmatism, especially if close and fast is more critical than long range accuracy – which is exactly what a patrol carbine is all about.

There are also some impressive designs in combat-tested, low power, prism scopes with lighted and etched reticles on the market.

The best solution of all is to see an eye specialist and obtain either an updated or corrected prescription.

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