Spatial Resolution – The Devil is in the Detail

It appears to us as if we see our complete surroundings in detail, focus and color all the time. And yet, at any one time we only see a very small portion of our environment in high resolution.
You can test this yourself by trying to read text or describe details in your peripheral vision.

Human vision is based on two basic types of receptors:

  • rods for low-light conditions which cannot discern colors
  • cones for well-lit scenes which allow color vision

Schematic of the Human Eye

Schematic of the Human Eye - based on schematic - public domain. SVG source

These receptors are located at the back of the eye, on the retina, and capture light coming in through the pupils. The two types of receptors are not evenly distributed over the retina, but rather the color-sensing cones are clustered around the center of the retina, while the rods are more dense outside the center.

Rod Cone Density

Density of rods and cones in the eye - CC-BY by Jonas Tullus. SVG source

Most of what we see is blurry

Only a very small part at the center of our retina – the fovea – has a high density of receptors which we use for high resolution focal vision, while our peripheral vision has lower resolution and is much better suited for detecting motion. What this means is that a snapshot of our visual system looks something like the below image, with a high-resolution, sharp center focal area and more and more desaturated and low resolution towards the edges.

Foveated Image

Original image (top), Foveated Image (bottom) - CC-BY, morgueFile license. Process I followed for foveation.

The fovea centralis covers ca. 1% of our visual field. As the retina is made up of an array of receptors, some projected visual details are smaller than the distance between two receptors and therefore cannot be detected.

The buzzard has a 6-8 times higher density of receptors in its eye and therefore can detect much smaller details from much larger distances.

Under ideal circumstances the human eye can resolve details as small as the thickness of a single hair, but in general suboptimal lighting, lens aberrations and other factors only allow much larger details to be discerned.

In summary:

  • we only ever see 1% of our field of view sharply, in focus and in color at any moment
  • the experience of an in-focus, colorful, persistent world is something that is created by our brain based on experience and expectation

This is part of the stream States of Mind - find previous and follow-up posts there.

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