Empty-field myopia is a quirk of vision: when the view has no reference points — a clear sky, an open field — the relaxed eye stops focusing at distance and settles on its own at roughly arm's length away.
The pilot is convinced they are watching the airspace, yet actually detects far less than assumed — a distant aircraft against an empty sky can go unseen. The countermeasure: periodically refocus on a distant object — the horizon, a treeline — to "reset" the eyes.