What is vision /20 if contacts are -10?!

What is vision /20 if contacts are -10?

my bf has very poor vision. his contacts used to be a -9.75, now its a -10.
anyone know what this is /20?

2 weeks ago
i found a chart after alot of lookin that said its 800... not sure it thats right or not. but i learned that after 200 (legally blind) you can get help paying for contacts etc, which would be really nice since his are so expensive, and he's always gotta have extras just incase.

First powers, then vision..

Powers:

The power of the lens is a measure of how much the lens bends light. The more powerful the lens, the more it bends light or the closer it bends light to itself.

Light moves from left to right (for this discussion). The light rays are parallel moving across the page. When they hit a window, like a house window, they keep on going straight through, no bending.

Now take a lens where the window pane is. It's a convex lens, meaning it is bent out from the center on both sides, like a magnifying lens or glass.

The center light ray will still be hitting a flat piece of glass, so the center light ray will not be deviated. But away from the center, the light rays will hit the glass at an angle and those light rays will be bent towards that center undeviated light ray and actually cross it some distance from the lens. Sort of like using a magnifying lens to focus the sun to a point to start a camp fire.

The power of the lens is equal to one over the distance in meters. or P=1/d.
So if the light rays hit the lens and are bent so that one meter away, on the right side they come to a focus, the power of that lens will be +1.00.

If the lens is more powerful than that and focus the light at half a meter away, that lens will be a +2.00 (P=1/.5)

If the focus is a third of a meter, +3.00
a forth of a meter, +4.00
a fifth of a meter, +5.00
a tenth of a meter +10.00

That's what those numbers mean.

This person is already focused at a tenth of a meter or about 4 inches in front of his eyes. And it is very clear there. He can see just fine at that distance. But anything beyond that distance is very blurry.

So since he's basically got +10.00 lenses IN his eyes, he has to put -10.00 lenses in front of his eyes to see at a distance like "normal" people.

Vision part.

The 20/20 thing is that most people see at 20 feet, a letter that is so high or so big. From the eye to that letter...on the top to the bottom..it forms an angle, which is about 5 degrees. That's as 'good' as a normal person can see. So if he moves back twice as far, say 40 feet, and looks at the chart, he'll need a letter twice as big to be able to tell what it is. That's the 40 foot letter.
Still, it's only 5 degrees as far as an angle goes.

At 100 feet, that's a pretty big letter. At 200 feet, even BIGGER.

So if someone stands at 20 feet from a chart (that's the top number...the distance from the chart) and can only see the 400 foot letter that normal folks can see way back there 400 feet away, his vision is measured as 20/400.

If the eye can see well, when corrected for distance with the right Rx, and it sees 20/20, it's a good eye. That it is focused nearer than normal when not corrected just means that the mechanism of focusing light for that eye is more powerful than normal, whatever the type (axial, corneal, lenticular...whatever the type of nearsightedness or myopia or shortsighted....you pick the word you like).

Or if farsighted, or hyperopic, those numbers will be + numbers to pull the focus of their 'weaker' eyes from way behind the eye up to the retina.