Just wanted to chime in, as
ProfessorCurly's unofficial 'consultant' (

), I disagree with
awesomeshotdude.
Your eyes' lenses must change considerably more in order to focus on objects up close than far away. This is because the closer the object, the more the eye needs to refract the light, while with objects at a distance, the light entering the eye is almost parallel.
Combat obscurants such as dust and smoke, targets at varying distant ranges, intervening terrain, and the sights of your weapon itself do not present the same significant change in lens shape as your
immediate surroundings do.
I'm not saying that focus won't be changing in combat. On the contrary!
I'm simply stating that your eye will be better at focusing on targets and picking out detail at range than having a lag-response and rapid requirement for change at close ranges.
@
Lion in the Stars, your marksmanship in a competition setting might be suffering because it is advised that you focus on the front sight tip/post of your rifle with your dominant eye open and your other eye shut, vice the target itself. The target should be a blur, with your aim at center mass/desired aim-point, and your eyes focusing on the sight alignment and picture of the firearm in question.
In combat, however, modern sights tend to favor keeping both eyes open and looking at a target, while placing a pipper or bead on the target and firing. Like I said previously, at a distance your eye has to adjust less in order to maintain focus on a target than the rapid and major changes in lens shape required in extremely close quarters combat, where your eye has to quickly adjust between your target, his various limbs, and the terrain behind him (anywhere from a wall 2 meters to his left to the building a half-kilometer down the street).
Your human focusing reflex in CQC is taken for granted when the enemy gets the jump on you. If he's moving fast enough, you may not even be able to identify that he's wielding a weapon such as a knife or baton until it strikes you. We're talking combat here, not sparring in martial arts, where you know your opponent's capabilities and what he's got to play with. In combat, you have to rapidly process a multitude of entirely
new sensory information about the target itself, and still defend yourself. Being unable to actually
see the target and process that he has a knife in his left hand, a pistol in the holster on his hip, and that there is another guy 10 feet away carrying who-knows-what - all within the span of perhaps the 15-30 seconds of combat - presents a significant tactical disadvantage for the one with the slower eyesight adjustments. All that information is absolutely vital to gaining a positional advantage on the enemy, and how you plan on engaging him.
All in all, this is relatively moot in a 40k combat setting, because the Tau himself is focusing on the lens of his helmet, with the optics doing most of the work.
Now that I have spoken my piece, I will fade back into the shadows.
