Nearsightedness is the result of the inability of the eye to correctly focus light at a distance, interlacing light coming from other directions with light from the targeted object, adding a "blur".

If a nearsighted person were to look at a small, bright object, a fire for example, from down a long, black and unlit hall, in theory there would be no light from other directions available to confound the image, and the image would be clear.

Does this mean that a nearsighted person could, in theory and in practice, identify detail at a distance so long as background "noise" light is eliminated?

  • Why not ask a nearsighted person? If your theory disagrees with what they tell you then your theory is wrong. (Your theory does disagree with what this nearsighted person will tell you.) – tfb Nov 23 '16 at 7:27

No it wouldn't. When properly focused, light originating from a particular point takes different paths into the lens, then all those paths converge to a single point on the focal plane.

In addition, light from any other direction is sent to a different point on the focal plane. This allows a 1:1 mapping from points in the field of view to points on the focal plane.

When the focus is off, the different paths that light takes from a point on the source to the eye do not converge and strike different points on the focal plane. In addition, these points may also be struck by light coming from other sources.

This divergence means a point source would activate a region on the plane, not a point. Eliminating other sources would reduce one source of trouble, but it is not sufficient for detailed imaging.

Your example of a fire is not a point source. It would have brightness differences over small distances. These differences would be smeared out as the focus is lost.

You can test this yourself. In an otherwise dark room or hall, you can image a bright window or television on the opposite wall with a magnifying glass at the correct distance. To simulate a myopic eye, just move the lens a little bit away from the wall.

I have had personal experience with this problem. The light from a fire is, as pointed out by BowlOfRed, not from a point source, so it would be "smeared out" on the retina, and appear as a blur to the myopic person. This can be improved by looking through a screen with a pinhole in it, whereby the screen is held very close to the eye. The smaller the pinhole, the less blurring effect you will see, because there is less light to create the smeared out image on your retina. However, as the pinhole gets smaller, the amount of light reaching the retina gets reduced, so the perceived image gets less blurred but also dimmer.

  • By the way, a point source is also not in focus for a nearsighted person. Also by personal experience, stars will be smeared out when I'm not using my contact lenses. As @BowlOfRed mentions in his/her first paragraph, a point source should converge to a single point on the retina. For a person with imperfect vision... it doesn't. – James Nov 23 '16 at 14:13
  • How is the black hall any different from a pinhole? You're essentially just eliminating all background light in a different way – TheEnvironmentalist Nov 24 '16 at 4:18
  • @TheEnvironmentalist: With the black hall, the light from the point source enters the entire front of the lens and does not get re-converged to a single point on the retina. For a pinhole, the light only enters on a small area of the lens, resulting in less smearing of light across the retina. – James Nov 24 '16 at 14:49

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