The limits of vision

Our eyes can do amazing things, but we need to keep in mind that they can do nothing without the visual cortex, which takes up about 30 % of the brain’s capacity and processes as many as two billion pieces of information each second. It is the combination of sensor (the eye) and processor (the visual cortex) that create what we call vision. Let’s just list some of the capabilities of vision:

Distance
The earth surface curves (the horizon) out of sight at a distance of about 3.1 miles (5 km) but our vision can perceive far beyond the horizon. The farthest star we can see with our naked eye is V762 Cas (brightness magnitude 5.8 ) in Cassiopeia at 16.308 light-years distance (9.5868623^16 miles), but if the night is clear and you know where to look, you can see the Andromeda Galaxy (M31), that’s 2.537 million light-years away.

Speed
The shortest moment a person can see a light source is based on Bloch’s Law, which defines the visual threshold that is reached when both illumination and time reach a constant. Simply said, there’s a balance point between the intensity and duration in a flash of light. As extremely birth light might appear the same when shown for a nanosecond as a dim light shown for a tenth of a second.

One study shows that fighter pilots (people with very good eyesight) can observe an image flashed on a monitor for only 1/250th of a second. Most people can see a flickering light source as steady illumination at a rate of 50 to 60 times a second (or hertz).

Resolution
The visual resolution of the human eye is about 1 arc minute (1/60 of a degree). At a distance of 20″, that’s about 170 dpi (pixels) – a dot pitch of around 0.14 mm. To translate this to something familiar, a 30″ monitor with a 16:9 aspect ratio would be sized around 26″ x 15″ and would need a resolution of 4400 x 2600 pixels to realize 170 dpi.

But there’s also angular resolution (the ability to distinguish two similar points that are close to each other). A simple example is car head lights, at a great distance they appear as one light, but at some point you are able to tell that there are two lights.

Angular resolution is measured in arc minutes (1/60th of a degree) and seconds (1/3600th of degree) of field of view. The angular resolution average human eye is one arc minute. For example, a one third of a millimeter wide line seen at arm’s length is 1 arc minute.

Foveal viewpoint
The fovea is the part of human eye responsible for sharp central vision (the only part of the retina that permits 100% visual acuity) The fovea is small, only about two degrees, and is necessary for visual detail like reading or tracking moving objects (hunting). Objects outside of the fovea are not in focus, even though our brains tell us they are.

fovea

Over the next few days I’ll concentrate solely on vision.

Web Development Application:

There are many applications for understanding vision. For example, it is important to keep the limits of the fovea in mind when writing because most readers scan before they read, and only see a limited amount of the page.

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