- January 31, 2016 at 3:34 am #22474Sgt. MikeParticipant
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This will be extremely lengthy and some what dry but before we consider the effects of light and sight errors we first must understand how the eye actually works. But I do recommend most emphasis is noted in the latter part of this post to get to the real answers one generally is looking for.
Aiming places great demands on the shooter’s visual capacity. Accuracy and consistency directly depend on visual acuity and the conditions that determine visual acuity. As the sensory organ, the human eye, coupled with the processing of the brain is capable of distinguishing millions of different colors, shapes, size, brightness and location of objects in the environment. We depend on this sense more than any other of the five senses, especially in the shooting sports, so it is important to know what we are dealing with as we aim. The eyeball is an optical device for focusing light. The front portion of the eye consists of several refractory tissues and surfaces, the cornea, aqueous humor, the iris, which has an opening called the pupil, the crystalline lens and the vitreous humor, through which light passes to stimulate a light sensitive membrane, the retina. The image formed on the retina, albeit smaller and upside down, is converted to nerve impulses by a photochemical reaction and transmitted to the visual cortex portion of the brain via the optic nerve that processes what we see. In order to see the world around us clearly, the image formed on the retina must be sharp. The normal eye at rest (or relaxed) is focused at infinity, so distant objects appear in focus, but nearby objects appear out of focus. To see closer objects clearly, the ciliary muscle contracts reflexively to change the shape of the crystalline lens to a more convex form increasing the refractive power and bring the near object into focus on the retina. This ability is called accommodation.
Distant objects will now be out of focus. The eye cannot clearly focus on objects located at different distances at the same instant. While the reflex action of accommodation can take place quickly (especially in younger people when the crystalline lens is quite flexible) this puts a strain on the visual apparatus and
must be avoided. The eye also responds automatically to varying amounts of
ambient light, by opening or closing the pupil via another reflexive action of the group of smooth muscles that controls the iris. This dilation or contraction of the pupil, called adaptation, regulates the amount of light entering the eye. The normal pupil opening ranges from a maximum of approximately eight millimeters to a minimum of two or three millimeters. As we age, the maximum opening decreases to six millimeters making it more difficult for older people to see at night. The pupil opening adapts much more quickly to increased illumination
(a few seconds) than it does to a decrease in illumination (a few minutes). Therefore, it is critical to avoid looking at brightly illuminated objects before or during shooting.
The action of the pupil is similar to the f-stop in a camera. The depth of field or the range of distances that the camera (eye) sees as being in focus increases as the f-stop aperture (pupil) size becomes smaller. The artificial pupil of the fixed rear aperture, or an adjustable rear iris, takes advantage of this thereby allowing the shooter to increase the depth of field so all the elements of the sight picture are clearly seen in focus.
In order to see the world around us clearly, the image formed on the retina must be sharp. The normal eye at rest (or relaxed) is focused at infinity, so distant objects appear in focus, but nearby objects appear out of focus. To see closer
objects clearly, the ciliary muscle contracts reflexively to change the shape of the crystalline lens to a more convex form increasing the refractive power and bring the near object into focus on the retina. This ability is called accommodation.
Distant objects will now be out of focus.
Imperfections of the Eye
As a result of optical imperfections of the various components of the eye, the edges of the image produced on the retina are not always perfectly clear. This is the ultimate limit of the resolution of the eye or visual acuity. However, under differing conditions of ambient light and/or fatigue, visual acuity is not constant and can change.
There are several inherent phenomena that occur as a result of the eye being an optical instrument. The first is spherical aberration, which occurs when parallel rays of light passing through the crystalline lens are refracted differently and thus not focused at a single place on the retina. Light passing near the edge of the pupil, farther from the center of the opening, are refracted more than those passing through the center. This appears as a circle of diffused light rather than a sharp image. Spherical aberration is at a maximum when the pupil is open
the most. Image clarity can be improved if one can eliminate the outer rays by either contracting the pupil or using an artificial pupil like that on the rear sight
A related problem is chromatic aberration. Visible light is made of all the colors of the spectrum. When visible light passes through a lens, light in the blue and violet region refracts more than that in the orange and red region of the spectrum focusing each color at a slightly different point, causing a fringe or margin of colors to appear around the edges of the image, especially around bright objects. This occurs because lenses have different refractive indexes for different wavelengths of light. Eliminating or reducing the amount of shorter
wavelength (blue) light by a filter can improve image clarity. Another phenomenon occurs when light passes through small openings, like the pupil or rear aperture, the rays bend and produce an image on the retina that is not a single point, but a circle surrounded by a number of concentric light and
dark rings of decreasing brightness. This is called light diffraction and is due to the wave nature of light. Diffraction rings are only noticeable when the pupil is very small and is the opposite of the cause of spherical aberration (large pupil).
Diffraction effects are more noticeable when bright light is shining into the eyes causing the pupil to contract. Another demonstration of diffraction is found when looking through a small aperture. Looking carefully at the center of the opening,
there seems to be a faint grey ring floating there, it is not dirt or fuzz, it is the diffraction pattern of the light.
Light can also be diffused as it passes through the various eye media, which are not absolutely transparent. Light diffusion manifests itself as a radiance or weakly luminous haze covering the field of vision, and is especially noticeable as a halo of light around brightly illuminated objects against a dark background or when bright light enters the eye directly. Light irradiation is probably more applicable to pistol shooting with open sights, but it is included here for those
who may use a post insert to show the effect of optical over estimation of sizes of objects set against a dark background.
A change in the brightness of the target’s white background will cause the eye to perceive the space between the front sight and the lower edge of the target as being different even though it is the same, resulting in high or low shots.
The eye is considered normal if the rays of light from a distant object entering the eye focus exactly on the retina without any effort at accommodation. However, other optical imperfections of the eye affect visual acuity including nearsightedness (myopia), farsightedness (hyperopia) and astigmatism.
Nearsightedness occurs when the parallel rays entering the eye are focused in front of the retina. This is typically the result of an eyeball that is too long or a lens that has too much refracting power. Myopia is easily corrected with proper corrective lenses. Many shooters are nearsighted and wearing glasses does not interfere with their performance.
Farsightedness is the opposite condition from myopia. An eyeball that is too short and/or a lens that has too little refracting power causes light entering the eye to focus behind the retina.
Shooters over the age of about 40, usually suffers from presbyopia, which is due to the gradual hardening with age of the crystalline lens so that it is no longer flexible enough for the ciliary muscles to change the shape sufficiently to focus on close objects. These conditions are a bit more challenging to correct but can be resolved by selecting appropriate corrective lenses. An eye where the cornea and the crystalline lens do not have a perfectly spherical shape is astigmatic. The light rays entering the eye do not form a single focused image on the retina, but rather several foci at various distances from the retina. This causes the image to be indistinct and erratic. Corrective lenses can also be used to fix astigmatism but it is important that the orientation of the lens in shooting glasses be maintained correctly. If the lens is rotated off the correct axis in relation to the eye, the shooter’s vision will be affected. Even if the eye and its structures are perfectly normal, the tear layer on the outside of the cornea can cause slight astigmatism that can be transient. Dry weather can reduce the amount of tear layer present at various places on the cornea and this can influence clear vision until the tear layer refreshes by blinking the eyelids.
- January 31, 2016 at 3:35 am #22475Sgt. MikeParticipant
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Every shooter should have their vision checked regularly with a thorough eye examination, and even small defects should be corrected. Over long courses of fire, the extra effort to accommodation will fatigue the eye with a deterioration of vision. It is also important that a corrective lens is placed so that the line of sight is perpendicular to the surface of the lens and through the center of the lens. This is because the center of the lens is ground more precisely to the prescription. Special shooting glass frames that can be adjusted to hold the lens in the correct orientation when the head is in the aiming position are essential once the shooter advances.
There is still more we need to know about the eyes and how they work including binocular vision and how to adjust and optimize the aiming aids available to the shooter.
Looking at how the USMC as well as the Services attacks teaches the differing light conditions they adhere to the following:
Discovering Different Light Conditions. Many shooters do not recognize that light conditions can affect their shooting accuracy and affect a weapon’s Zero. A change in light condition, which may not be noticed, this can cause your aim at what you think is the correct aiming points, but really are not. What appears to be center mass on the target may in fact be several inches higher or lower, left or right. The zero on your rifle may need to be adjusted to compensate for the effects of changing light conditions. Maintaining a center mass hold, regardless of how indistinct the target appears, ensures the best chances for an effective shot.
(1) Bright Light. Bright light conditions exist under a clear blue sky with no fog or haze present to filter the sunlight.
(a) Affects On The Target. Bright light can make a target appear smaller and farther away. As a result, it is easy to overestimate range.
(b) Affects On The Sights. Bright light shining from above makes the front sight post appear shorter and bright light from the side makes the front sight post appear narrower. This affects aiming because your shooters will aim at center mass using the perceived tip of the front sight post, which is altered due to the effects of light and will alter their aiming point.
(2) Haze. Haze exists when smog, fog, dust, or humidity is present. Haze is not bright, but it can be uncomfortable to the eyes. Haze can make a target appear indistinct, making it difficult to establish sight picture.
(3) Overcast. Overcast conditions exist when a solid layer of clouds blocks the sun. The amount of light changes as the cloud cover thickens. Overcast conditions make a target appear larger and closer. As a result, it is easy to underestimate range.
(a) Light Overcast. Light overcast conditions exist when no blue sky is visible and a thin layer of clouds is present. In light overcast, both the target and the rifle sights appear very distinct. Light overcast is comfortable on the eyes with no glare present, making probably the best light condition for shooting.
(b) Dark Heavy Overcast. Dark heavy overcast conditions exist when the sky is completely overcast with most of the light blotted out by the clouds. As the overcast thickens, it becomes difficult to identify the target from the surroundings.
(4) Scattered Clouds. Scattered cloud conditions exist when the clouds are broken up into small patches with the sun appearing at times between the clouds. Your shooters eyes may have problems adjusting between a target which is brightly lit and one that is shadowed.
(5) Moving Clouds. Moving clouds exist when scattered clouds move across the sky rapidly, making the sun appear periodically. Rapidly moving clouds can fatigue the eyes due to the rapid changes from bright light to shadows. This condition is probably the most difficult to contend with because the light changes rapidly. If the situation permits, this condition can be compensated for by selecting one of the two light conditions (bright light or shadow) in which to fire. Best results will be obtained if each shot is fired under the same light condition.
(6) Recording Light Conditions In The Data Book. A significant change in light condition should be recorded in the REMARKS block of the data book. This information will help determine how the type of light condition or change in condition affects your zero.
(Source http://www.lejeune.marines.mil/…/CMC-13%…Leture on the effects of weather upon the Rifleman)
The below text now is geared toward the scope user hence it seems to contradict the above text which is wrote for Iron sights
Light doesn’t directly affect bullet trajectory, but it changes the way you see the target through the telescopic sight. Essentially, you perceive the target in a different way, e.g., larger or smaller, and/or in a different place, depending on the sun’s position and light intensity. This leads to aiming errors. Every shooter faces these errors, but not all may have realized the underlying cause.
Although the cause of phenomenon is a subject of debate (just search through any forum you’ll find it), some attribute it to the light phenomena of refraction and diffraction. Unfortunately, there’s not a codified compensation method or a “rule of thumb” to deal with light. In many a text, you’ll find only a short paragraph regarding light effects on long range shooting, stating that the only way to manage it is to keep a record book with annotation of the light conditions for every shooting session you do, and use it to develop your own methods.
Comparing results of other shooters. What is consistent up to this point, confirmed by other shooters, is that light affects point of aim both on vertical and horizontal planes usually in this manner athough your observations may vary to a degree:
On the vertical plane, light intensity can lead to aiming error. When shooting with low intensity of light, compared to the light intensity while zeroing the rifle, the shot goes high. On the contrary, when shooting with greater intensity of light, compared to the light during rifle zeroing, the shot flies low. Generally use an adjustment of about ½ inch at 100yds and about 7in at 600yds.
On the horizontal plane, changes in light direction can lead to aiming error. Basically, there is a sort of illusion effect associated with from which direction the light is travelling. If the light comes from the shooter’s right, the shot lands left, and vice versa. I’ve not been able to quantify the exact amount of horizontal error because I always shoot in windy conditions. It’s virtually impossible to tell wind defection errors from light angle errors. Usually adjust about one click, or 0.1MIL, to the opposite direction of the light.
- January 31, 2016 at 1:16 pm #22504Alphawolf45Participant
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By how much could changing light conditions effect shots at just 100 yards and less?
- January 31, 2016 at 1:39 pm #22505GoodsteelKeymaster
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I think it depends on the shooter Steve. I know shooting from a covered area, with the light slightly behind is the most perfect situation for me. I was out at the Benton long range shooting at the 400 yard plate, with my M1A and I was holding a group the size of a dollar bill.
Here’s the plate. The other hits below were caused by my buddy’s 300BLK. Notice the shadow of the plate on the ground behind.
Now, in contrast, the next weekend, I was shooting in an open field feeling all confident and good about myself. I was shooting at a posted target 300 yards away, the sun was high and beating down on us. The sights had a lot of glare on them from the sun, and they were all blurry. I had a hard time just keeping it on paper.
After that was when Sarge introduced me to aerosol sight black. A quick spritz with that stuff and glare has a hard time getting a foothold. Still, the location of the sun and how it hits the sights is a frustrating thing to learn to deal with because it changes your perception of where things are (almost like Mirage).
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