Cost: [-15]
One Eye is a disadvantage. It can reflect creatures with 1 central eye.
Given that it can reflect beings with 2 eyes who lose one, consider Wounded to reflect that injury, and some limited form of Restricted Vision which only applies to one side.
A cyclops with a central eye would have more peripheral vision on his left than a human who lose his left eye and was relying on his right eye to see!
Controllable Disadvantage[]
Tactical Shooting mentions under Light Adaptation for Shooting in Darkness a trick for using this to offset the extra Adaptation Time penalties (closing an eye to get it used to darkness to avoid the extra -2 penalty)
However given there's a -3 penalty for using one eye, the incentive isn't really clear.
Test[]
Alternate between closing eyes and looking left and right with the open one to test it. With nose fixed forward, at the most the left eye can see into the front-left hex and the front-centre hex, while the right can see into the front-right and the centre hex.
Neck rotation can only add 90 degrees to this, but neck movements are not accounted for under helmet types. As 2014 points out in http://forums.sjgames.com/showthread.php?t=123292
- Class 2 gives No Peripheral Vision
- Class 3 and 4 give Tunnel Vision
That comes solely from restricting the eyes, even though these helmets say nothing about restricting neck movements. So neck movements are not part of it.
Neck movements would be like Reciprocal Advantages innovated with Using Abilities at Default properly enhanced with Reduced Fatigue Cost so as to be free. You would gamble your DX or HT to make them so that if it crit failed you would temporarily lose them, reflecting the tweaking of a neck improperly. Using Extra Time outside combat it would be very easy.