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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.