I think there was difficulty switching between nvg exterior vison and 'under the goggles' interior vision. This could have been compounded by that eyesight defect known as 'lack of accommodation' which is difficulty in focussing near-and-far which as those of us over fifty years old know creeps in gradually and finally locks us into fixed-focus machines. A few lucky people, myself included, have different focal lengths in the two eyes, but for most folk the fls are the same for both eyes and lack of accommodation hits unexpectedly hard. In a helicopter nvg situation, corrective spectacles would have to be very cunningly designed to be of assistance.