The Eye and Vision — Page 77, Lesson 120

I want to walk you through the human eye and vision, starting with a remarkable comparison that puts the eye's sensitivity into perspective. The human eye has about 1.2 million neurones — nerve fibres — leading from the retina back to the visual cortex of the brain. By contrast, the inner ears only have about 50,000 neurones. That means the eye is roughly 24 times more sensitive than the ear. So right away, you can appreciate just how much information your visual system is designed to process. Now let's focus on the key structure that gives us sharp, detailed sight: the fovea. The fovea is the central part of the retina. What makes it special is that it is composed only of cone cells — the photoreceptors responsible for colour vision and fine detail. Whenever you want to examine something closely, your eye automatically moves so that the image falls on the fovea. The rest of the retina, outside the fovea, serves a different purpose: it attracts your attention to movement and change in your surroundings. But only at the fovea do you get what we call 20/20 vision, or in metric terms, 6/6. This is termed central vision. Your entire visual field is made up of two parts: central vision, which comes from the fovea, and peripheral vision, which comes from the rest of the retina. There's an important note here: alphanumeric information — that means letters, numbers, and symbols — is limited to the foveal area. So when you're reading a checklist or an instrument, you are using your fovea. Let's define visual acuity. Visual acuity is a measure of central vision. The numbers like 20/20 or 6/6 are a way of measuring it. More precisely, visual acuity is the ability to discriminate — to tell apart — details at varying distances. If someone has 20/20 vision, it means they can see at 20 feet what a so-called normal person can also see at 20 feet. In metres, that's 6/6. Now, if the figures are 20/40, or 6/12, that means the observer can read at 20 feet what a normal person can read at 40 feet — or in metres, at 6 metres what a normal person can read at 12 metres. So the larger the second number, the worse the acuity. Here's a critical point for you as a pilot: any resolving power at the fovea drops rapidly as the angular distance from the fovea increases. At just 5 degrees away from the fovea, acuity drops to 20/40 — that's half as good as at the fovea. At approximately 25 degrees away, acuity decreases to a tenth — that's 20/200. So your sharp, detailed vision is confined to a very small central area. This has huge implications for scanning instruments and looking for traffic, which we'll build on later.