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.
Your eyes does not get light only through a tiny point but through the entire pupil. The light from an object that hits the left side of your pupil comes inn at a different angle then the light from the same object that hits the right side of your pupil. The difference in angle depends on how far that object is from you. So depending on how far it is you need to adjust your lens differently so that the light that hits either side of your pupil hits the retina in the same spot.
Your eye can’t focus on things properly.
So most commonly, either you can see things in the distance fine and close stuff is blurry. Or you can see close things and far stuff is blurry.
I can’t focus on things far away, so I need glasses to help. So I’m nearsighted.
It’s got something to do with how my eye adjusts it’s shape and it’s focal range. It can’t do a certain thing very well.
Has everything to do with your eyeshape and condition of your retina/cornea. Look up myopia and hyperopia.
The distance the light travels isn’t the issue. The issue is how much the lens has to move to focus the light. It is like having a camera with a broken focus ring that can only move along a portion of the range it is supposed to. You have no problem focusing in the range it can still move in, but anything outside that range can’t be brought into focus.
Nearsightedness (Myopia) and Farsightedness (Hyperopia) are both caused by the eye being misshaped.
With Myopia, the eye is elongated front-to-back, while with Hyperopia, the eye is squashed front-to-back. In each case, the issues with vision arise when the lens cannot focus the image on the retina properly because the retina, in effect sits outside the range the lens is capable of focusing in; in the case of Myopia, the image is focused in front of the retina, in Hyperopia, it’s focused behind the retina. The lens in effect is designed to focus the image in only one place, so if the retina isn’t where it’s supposed to be, there are problems.
Both Nearsightedness and Farsightedness can also be caused by the lens of the eye having too much (Myopia), or not enough (Hyperopia) curvature, which results in the same effect.
Each condition effects distance or close-up vision respectively.
Latest Answers