So I understand we see things because light is reflecting off an object and that light is hitting our eye. It gets focused by our eye onto our retina. So it makes sense to me how someone can have bad eyesight, where everything goes equally out of focus. But how can a person be near-sighted, where light travelling a shorter distance (ie reflected from a closer object) is in focus but light travelling a farther distance is not in focus? And the reverse for far-sighted? How does the distance the light is traveling affect whether it can be in focus? It seems to me like everything should be able to be equally in focus, or equally out of focus
In: Biology
The retina is a 2D plane surface, and the World is 3D. The lens in the eye is controlled by muscles to select a part of the World to focus on the retina. Depending on the size of the lens opening (or the aperture) a range of distances, called the Field Depth, is focused and the rest is blurry. This is a lens geometry limitation, not a visual perception problem, you can see depth of field changes when you watch a camera try to auto focus. If you hold your fingertip almost touching the lens on your smartphone, you’ll see its quite blurred.
Near-sight and Far-sighted are terms that describe malfunctions in this controlled focus process. A near-sighted person can’t push the field out as far as they’d like to, so distant objects are always blurry. They can focus just fine up close, and sometimes even closer than “normal”. Similarly, far-sighted people can see fine at distance, but can’t shift the field close enough to read a book they are holding. You often see this as you age, with people holding the newspaper farther from their eyes until their arms aren’t long enough and they have to get reading glasses.
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