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This is potentially more or less a question about optics, but I've never been truly satisfied with any explanation that I've been given about Myopia. In fact, I almost can't even believe it. There are a million different versions of this diagram:

enter image description here

I understand that images of things could appear blurry when those things are focused in front of the retina, but then what discriminates between near and far things? Shouldn't everything be blurry? What exactly is different about this image when the thing being viewed is in close proximity to the eye?

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The image you gave is not clear because it does not show the fact that the divergence pattern of light from a close object is different from the divergence pattern of light from a distant object.

enter image description here

Light rays from a close object (first image) diverge, and therefore they are in focus for the myopic eye.

Light rays from a distant object (second image) is approximately parallel, and therefore they are not in focus for the myopic eye. The lens corrects for this inability to focus and allows the distant object to become in focus.

Also note that the myopic person is still capable of focusing on close objects despite the lens because the lenses are still capable of dynamic adjustment, but less so than a person with perfect eyesight. You can test this by focusing on a near object, then putting on your glasses. The object should be initially out of focus, and then become in focus as the eye lenses adjust to the distance.

Myopia/presbyopia is fundamentally an issue of the focal range of the eye becoming limited to a specific range. If the ability of the eye lenses to dynamically adjust is further weakened, it may require progressive lenses or bifocals to correct the issue.

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  • $\begingroup$ This is a great answer! I read originally that it was because of the angle of incidence of the photon and I didn't believe it for the following reason: if a 4m tall statue stands a large distance away from me, it's blurry. If I have a 10cm version of the same statue and I hold it at a distance away from me so it appears the same height as the statue in the distance, it's still sharp. In that scenario, the light from both statues is parallel(or so I thought) but NOW it makes sense, since the incident photons are still going to be at an angle because they're coming from ALL points of the statue. $\endgroup$ – Abe Fehr May 15 '15 at 1:33

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