system
Given the following post, respond as if you're commenting on Reddit:
human
What does 20/10 vision mean?

I just had a doctors appointment a few weeks ago and got a score of 20/10 on the eye test. I know the "average" score is 20/20, so does that mean my vision is twice as good as a normal persons? And what IS twice as good? 150 megapixels? Really curious.
gpt
The standard eyechart ("Snellen") test is really a very blunt tool for quantifying vision. It is reasonably good at determining your ability to resolve detail using your central vision which, while important, is just one component of visual performance.

To directly answer you, 20/10 means that you can resolve details at 20 feet away what the hypothetical "average" person needs to be 10 feet away to see. Healthy young adults can fairly routinely see 20/15 or better. As you age, your ability tends to diminish, sadly.

In visual acuity research, if we really want to get down and dirty with examining resolving fine detail, we use tools like spatial gratings, which tell how many sinusoidal-like transitions of white-to-black-to-white bars can be detected. We measure the acuity in cycles/degree; higher means more resolving power.

I am leery of the whole attempt to relate human vision to megapixels, because we do not have uniform function across our retina. In fact, only the most central one-degree diameter circle is capable of resolving fine detail. This is the fovea, where the photoreceptors are most densely packed, like stadium seating on multiple concentric levels. This trick allows more visual fields per area. Some birds of prey, raptors, have even deeper foveal pits, allowing better packing of receptors, and therefore better acuity.

But vision is way more than just the retina. Your lens and cornea, and all the juices and jellies in your eye must be properly functioning to allow you to see well. When we are young, our lens is clearest and most flexible, so it can be shaped (squashed or flattened) to bring images to your retina. As we age, our lens opacifies and also loses the ability to accommodate (change shape).