Only known game to do this correctly to date is R6 Raven Shield...

Since ARMA 3 is a lifelike game, all things should be analysed from real life.

This really isn't necessary. It offers absolutely no benefit whatsoever to reduce the viewing space to a circle. Infact, it would be worse than the viewing angle you would have in real life. Displaying a circle would indicate you are looking through a telescope. Single lens. When you look through binoculars, each eye is magnified and your brain merges the two together, so you maintain depth of field, and a larger viewing angle. Its just not needed in the game. It offers no benefit, and not does not offer any advantage.

concider quad head night visions used by the seals on that night in obotobad, sadly you're wrong, During my time in the military I've seen various night vision with various periphiral capeabilities, be it monocular or tripple or quad headers, the exit lense can be any shape from oval to round to square to hexagon, all have good and negative points.

but it doesnt always have to be round.

While the vote on the thread speaks for itself -- as obviously many voting here have actual experience with the instruments described, apprehend the facts correctly, and don't use unsupported evidence to make a case; the two subjective posts above should be addressed to avoid vote confusion; though I'm not in any way criticizing personal preference...

The 'benefit' of correctly converged optics shadow mask include that (contrary to samsamm777's comment) a circular shadow mask is technically correct, more realistic, more authentic, and is easier to scale to the FOV limitations of the game across different resolutions and aspect ratios. The 'advantage' offered is scale is preserved in the game across all mechanics of unaided, monocular/scope, or binocular optics and the correct converged shape is used. 'Advantage' as in out of scale 'advantage' can and should be obviated and is easier to scale correctly (and fairly), across more displays and resolutions using a round mask.

Inimical_rize's comments about quad tube NVG like the L-3 GPNVG-18 are also technically wrong, when you look through a system like the L-3 GPNVG-18 you are primarily looking through the two center tubes that like all NVG have to be properly converged to be useful. Looking forward you're view is quite conventional and appears as a circle, with two 'side view' mirror like images that you really have to turn your eyes to properly see or use.

Panoramic NVG are impractical; they: weigh more then twice the equivalent binocular NVG, have less then half the battery life per unit weight of binocular NVG, the peripheral optics are of only marginal value adding useless exhausting mass and inertia to an already overly encumbered helmet, a rifle can not be aimed through the peripheral optics, and the added optics constrict the Operator's pupils with useless stray light impairing acuity. Most importantly panoramic NVG are not depicted in ArmA III (so far) so are not even germane to the tracker article.

Reference:

http://tnvc.com/shop/l-3-gpnvg-18/

The direction more advanced NVG optics are taking is along the lines of the O'Gara Group AN/PVS-21; these are very compact, very rugged, long endurance, low mass, low inertia (mass is closer to the Operator's face), and achieve very useful non distorting high resolution FOV of 40° that don't constrict the Operator's pupils with excessive useless illumination.

Reference:

http://www.sts-eo.com/ANPVS21.php