Just A Kid From Cleveland

Me after endgame:

in a rough nutshell:

a normal eye (emmetropic) should focus light directly on your retina:

[_____X)

where:

[ = cornea & lens

X = image formed by the eye focusing light

) = retina

if your eye is too long or your eye’s refractive power is too strong, the image is focused on an area in front of the retina and you are myopic:

[___X__)

if your eye is too short or your eye’s refractive power is too weak, the image is focused on an area behind your retina and you are hyperopic:

[_____)  X

specially curved lenses of various compositions and powers bend light into your eye in a way that helps to correct your eye’s refractive error by moving that focused image forward or backward so that it falls directly onto the retina. a concave lens (minus power) diverges light, moving the image away from the eye’s lens, and a convex lens (plus power) converges light, moving the image towards the eye’s lens.

minus lens: )_(

plus lens: (_)

[___X__) + (minus lens) = [_____X)

[_____)  X + (plus lens) = [_____X)

a sphere lens focuses light in all degrees of the lens. the vertical meridian of a lens focuses light horizontally, and the horizontal meridian of a lens focuses light vertically; both are 90˚ apart. if you have astigmatism, these two meridians are different powers, meaning one meridian is focused in front of the other. 

[___/__)  \

this is corrected with a cylinder lens that has one meridian with no curve (the axis), and the other with a curve in order to bring the two meridians together, positioned on the right axis as the meridians are not always 180˚ and 90˚, you may have an axis of 45˚ or something. together with the sphere lens to make sure the two meridians are right on the retina, astigmatism can be corrected.

For instance, this is how you would correct a compound hyperopic eye:

[_____) /  \ + (correcting cylinder) = [_____) X

[_____) X + (correcting sphere) = [_____X)