What is that ?
Myopia is when the eye is to big for the cornea and lens can make a clear picture on the retina. This will result in at you can’t see clear far a way or if you shall use minus 1.00 D glasses you can see clear up to 1m/3feet. Or the eye can be a normal size but the cornea and the lens are stronger than normal, and make the clear photo in the eye in stead of on the retina. To correct this we put glasses or contact lenses on with minus that make the photo to appear clear on retina. This refractive error usually becomes clear to the person right a way, children can’t see the chalkboard at school or they sit too close to the tv, or they complain about blurred vision
Hyperopia is when the eye is to small and cornea and the lens make clear photo behind the eye. Or the powers of the cornea and lens is to weak and the eye has normal size resulting in the clear photo to be behind the retina. To correct this we use plus lenses in glasses or contact lenses. This kind of refractive error is often named the forgotten refractive error because of the lens ability to correct the error. Children are able to make op to and over 20D plus with the lens in the eye so they can see clear at all distances, but the child with about 6 D plus uses much energy to read and can have slow reading ability or is reluctant to work in close range. Many have small plus , with less than 4D they can live with their refractive error until they hit 40. Then they start using reading glasses, this is the age when presbyopia comes to live and many become amazed about that fact that they need both reading and farsighted glasses. They have had perfect sight all their lives without glasses.
Presbyopia is when the lens in the eye can’t make enough plus to read close up, it hits both hyperopic and myopic people, but the hyperopic will use reading glasses sooner than same age myopic person because of the eye refractive error helps myopic people to read closed up, they just take the glasses of to read.
Astigmatism is a refractive error that is combined with myopic or hyperopic. The astigmatism comes out because the eye is not totally round, and to understand this difference is best to use rugby ball and soccer as examples, as the eye is like a soccer and has the same refractive error any where in the photo. But if you have an eye like a rugby ball you got one axe that has more powerful refractive error than the other so you get two photos, one closer to retina and one 90 degrees angel more far from the retina. This type needs 2 different powers in the single glass or contact lens to join the both photos to one clear photo on the retina. This is not pleasant to correct at the first time but gets better after the brain and eyes get used to the corrections and should be corrected if over 0,25D.