Several things can make it longer — Page 88, Lesson 133

I want to walk you through the factors that can extend the time it takes for your eyes to detect an object. We just covered that the basic visual detection time is roughly one second, but that's under ideal conditions. Several things can make it longer. First, poor atmospheric conditions — haze, fog, smoke, or rain reduce contrast and scatter light, so your retina needs more time to register a target. Second, darkness. At night or in low light, your visual system shifts from cone-dominated photopic vision to rod-dominated scotopic vision. Rods are more sensitive but slower to respond, so detection time increases. Third, the size and contrast of the object. A small, low-contrast object against a similar background is much harder to pick out than a large, high-contrast one. Your brain needs extra processing time to separate it from the background. Fourth, and this is a key one for aviation, the angular approach of the object. This describes the path the object takes across your visual field. The excerpt gives a specific example: an aircraft approaching head-on — coming straight toward you — will stimulate the retina less than one tracking tangentially across the visual field. Why? A tangential target moves across many different retinal cells, creating a strong changing signal that grabs attention. A head-on target stays at roughly the same point on the retina, producing a much weaker stimulus, so it takes longer to detect. Now let's move to a new topic: monocular and binocular vision. Binocular vision means using both eyes together, with overlapping fields of view that give you depth perception through stereopsis. The key point here is that binocular vision is not essential for flying. There are many one-eyed — or monocular — pilots currently flying with a Class II medical certificate. A Class II medical is the standard for commercial pilots who are not operating in multi-crew operations requiring a Class I. However, there is an important qualification. If a pilot loses an eye, it normally takes some time for the brain to learn to compensate for the loss of binocular vision. The brain has to adapt to judging distance and speed using only monocular cues — things like relative size, motion parallax, and perspective. Only after that adaptation period can the individual regain their medical certificate. But here is the hard limit under EASA regulations: a person with vision in only one eye cannot be accepted under EASA as fit to fly. That means you cannot begin flight training or obtain an initial medical certificate with monocular vision. The allowance for monocular pilots applies only to those who already held a medical certificate and then lost vision in one eye, and who successfully adapt. So to summarise: binocular vision is not required to keep flying if you lose an eye and adapt, but you cannot start flying with only one eye under EASA rules.