And is there any way to convert one to the other? I’m not sure how they’re each affected by things like astigmatism. Thanks in advance for any answers!
In: Biology
The -6 value is in units of diopters. This is a unit of lens refraction, not really about visual acuity.
The 20/20 is a measure of relative readability. There is a relationship, ignoring astigmatism. -1 diopters is 20/40; -2 is 20/80; -3 is 20/160 (usually 20/150 because that’s a row on the chart); -4 is 20/300. Beyond that, things get a little more complex because the myopia requires more correction to compensate for the relatively constant distance from the eye to the lens.
Visual acuity and prescription are measuring different things.
Visual acuity measures the smallest detail you can see at a fixed distance (20 feet, hence the 20 part).
The prescription is how far forward or back your eye’s focus point is compared to where it should be. For example with shortsightedness your entire focus range is shifted closer, so rather than being able to see from infinity to 25cm you might be able to see from 50cm to 5cm. In these cases glasses actually hurt your close distance sight, but usually it’s more useful to be able to see things further away!
The reason there are these separate scales and you can’t convert between them is because you can have one without the other. For example you could be neither short nor long sighted but have a retina diseases that gives you bad visual acuity (no prescription could fix this). Or you could be long sighted so while you have 20/20 vision (visual acuity measures distant viewing) you still need glasses because you can’t read anything close to you.
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