how does refractive error work?

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I am slightly confused how refractive error really works and how it’s tied to visual acuity.

For example let’s say someone has 20/20 vision (both distant and reading), I totally understand that this person may still have a refractive error (plus or minus) due to the fact that their eye ball is still misshapen and the reason they have 20/20 is because their eyes manipulate the lens to focus light. That’s why doctors have to dilate your eyes; to take the eye muscles out of the equation. I guess my question is if a person can have 20/20 vision with some refractive error, is it possible people can have no refractive error and still not have 20/20 vision, say 20/50? How does that even work?

To follow up, I know pupil dilation affects people differently, near sighted people are not able to see things from a distance and the opposite is true for farsighted people. Would that mean that someone with no refractive error would be able to see clearly, distant and near, regardless of dilation?

In: Biology

> That’s why doctors have to dilate your eyes; to take the eye muscles out of the equation.

Not so. They dilate the eye so they can look in better, they check your vision with your eyes not dilated because the eye muscles are important to how your eyes normally operate. After all when you are actually using your glasses/contacts your eye muscles will be working.

> I guess my question is if a person can have 20/20 vision with some refractive error, is it possible people can have no refractive error and still not have 20/20 vision, say 20/50? How does that even work?

…Yes, but mainly because there are ways to have poor vision other than focus problems. Someone could have retinal degeneration due to various diseases that leaves them with poor vision despite no refractive error. Or perhaps their lens is foggy such as with cataracts, or they have trouble pointing their eyes in the same direction, etc.

> I know pupil dilation affects people differently, near sighted people are not able to see things from a distance and the opposite is true for farsighted people.

That isn’t due to pupil dilation. The focus of the eye isn’t performed by the iris opening and closing the pupil, there is a flexible transparent lens that is tugged on by muscles in the eye to change its shape and alter focus.

> Would that mean that someone with no refractive error would be able to see clearly, distant and near, regardless of dilation?

No, focus is still a thing when there is no error.