I went way down, out of the pattern. The tower called me to ask what was wrong. When I got in, I decided I couldn't fly again. I could see clearly only what was straight ahead of me. The disease first affects a doughnut-shaped area, surrounding the center of the retina, where photoreceptors called rods predominate.
Rods can respond to dim light, and thus make night vision possible.
The retina has 20 times more rods than cones, but when the rods falter, the retinal environment changes, with consequent damage to the cones. It isn't the first punch that disables you, but the combination.