While the same problems are encountered when driving cars, two other effects amplify these visual challenges for pilots. Empty field myopia is the tendency for the eye to focus in at 6-10 feet (2-3 metres) when there are no obvious objects to look at. When we look out at an indistinct background in the sky we can quickly become “near-sighted”. The second factor contributing to less accurate focusing is how wide our iris or the shutter in the eye is open. Under low light conditions, the iris of the eye tends to open wide and the consequence is that the depth of field of our focus is smaller. Under these conditions it is quite tiring for the eyes to focus properly, such that we tend to lapse into the same effect as with empty field myopia and not focus more than 6-10 feet (2-3) metres away. Now many will state that daytime pilots shouldn’t worry about this as they fly in bright conditions so that their eyes are “stopped down” (small iris opening) which should provide good depth of visual acuity. This would likely be true if pilots did not wear darkly tinted “aviator’s” sunglasses. Often our attempts to cut down on glare and look like “real” pilots can have side effects that we haven’t bargained for.
To put these problems in further perspective, consider the real flying situation. Even if you see an object it generally will take you at least one second to recognize that it is a hazard and another second to decide to turn to avoid the problem. The aircraft will at best require two seconds to be banked into even a medium turn and another three seconds to change course by 45 degrees. Under the best of conditions then collision avoidance will require 7 seconds. Modern gliders are often travelling around the countryside at 100 knots, which represents about 160 feet per second or recognizing objects at least 1000 feet away. This assumes that you are relatively stationary, perhaps represented by thermalling. For head-on situations the safe distance doubles if you are both closing at 100 knots.