One of the things I frequently dwell on is the ability of people to see their targets at Boomershoot. My informal tests concluded I could see 1 Minute of Angle (MOA) objects with good contrast and lighting. An email from a list I’m on confirmed that conclusion:
Contrary to common perception, 20/20 vision is not “perfect eyesight.”
It is common to refer to 20/20 vision as being “perfect eyesight,” but by definition it is “normal” visual acuity. 20/20 means your eyes can resolve a spatial pattern separated by an angle of one minute of arc. Put another way, 20/20 vision signifies that you can see at 20 feet (the first number) what a normal person can see at 20 feet (the second).
However, it is common for young people to have visual acuity around 20/15 or 20/12. Recent innovations in corrective eye surgery suggest that 20/10 acuity, or even 20/5, may be possible.
Visual acuity depends upon how accurately light is focused on the retina (mostly the macular region), the integrity of the eye’s neural elements, and the interpretative faculty of the brain.  “Normal” visual acuity is frequently considered to be what was defined by Snellen as the ability to recognize an optotype when it subtended 5 minutes of arc, that is Snellen’s chart 20/20 feet, 6/6 meter, 1.00 decimal or 0.0 logMAR. In humans, the maximum acuity of a healthy, emmetropic eye (and even ametropic eyes with correctors) is approximately 20/16 to 20/12, so it is inaccurate to refer to 20/20 visual acuity as “perfect” vision. 20/20 is the visual acuity needed to discriminate two points separated by 1 arc minute. The significance of the 20/20 standard can best be thought of as the lower limit of normal or as a screening cutoff. When used as a screening test subjects that reach this level need no further investigation, even though the average visual acuity of healthy eyes is 20/16 or 20/12.
Normally visual acuity refers to the ability to resolve two separated points or lines, but there are other measures of the ability of the visual system to discern spatial differences.
Vernier acuity measures the ability to align two line segments. Humans can do this with remarkable accuracy. Under optimal conditions of good illumination, high contrast, and long line segments, the limit to vernier acuity is about 8 arc seconds or 0.13 arc minutes, compared to about 0.6 arc minutes (20/12) for normal visual acuity or the 0.4 arc minute diameter of a foveal cone. Because the limit of vernier acuity is well below that imposed on regular visual acuity by the “retinal grain” or size of the foveal cones, it is thought to be a process of the visual cortex rather than the retina. Supporting this idea, vernier acuity seems to correspond very closely (and may have the same underlying mechanism) enabling one to discern very slight differences in the orientations of two lines, where orientation is known to be processed in the visual cortex.
So, if you have fine straight lines in your scope and are aligning with another line on your target you can align things about 4.5 times as accurately than you can with a scope that doesn’t have straight lines (think “post” type reticles) or targets with straight lines.
I’m wondering if this accounts for some of the increased consumption of the square targets we have been using the last couple of years compared to the round targets we used previously.
This tidbit has obvious application to snipers, and perhaps less obviously, to their camouflage.