Emmetropia, or what optometrists refer to as the perfect eye, is an eyeball that can see 20/20 at both distance and near with no assistance from glasses. Light that enters this eye will land perfectly on the retina allowing patients to see clearly at both distance and near. This is the eyeball that we all wish we had but unfortunately not all of us are that lucky. Every eyeball from here on out will be compared to an emmetropic eye because they are “perfect”.
Myopia, or more commonly known as nearsightedness, is condition in which we can’t see that well at distance, but we can see fairly well at near (as long as you don’t have a super high prescription). This can be for one of two reasons. In some people our eyes are just too damn long. They never stopped growing and now when light enters the eye, it doesn’t land on the retina like it normally would in an emmetropic eye, it lands in front of the retina. Since the retina is further back, optometrists prescribe minus lenses that will diverge the light a little further to allow it to land on the retina. The other reason why a person might be myopic is because they might just have too much refractive power in their eyes. Basically, light will still land in front of the retina even though their eyes are the same length as the “perfect” eye. To correct this eye doctors will still use minus lenses to cancel out the extra plus power the eye is creating.
Hyperopia, or farsightedness, is a little more of a tricky situation. Depending on high your prescription is, you might be able to see clearly at both distance and near or not well at either distance. The reason for this is because our eyes have a built-in focusing system that eye doctors call the accommodative system. To see up close we need a little bit of plus power. Anatomically, we have a lens that controls where light is focused in our eye. The lens is the key factor in this focusing system. To see up close, the muscles attached to our lens will contract and change the shape of the lens. This gives us the little bit of plus power we need to see up close. However, this is one of the big reasons why hyperopic with small prescriptions do not adapt well to glasses or don’t even know they need glasses until later on in life The accommodative system will make up for the small amount of plus power a hyperopic patient needs so that they can see clearly up close and far away.
Patients that need more plus power than our lens can give us face a different issue. These patients usually cannot see well at distance or up close because of where light entering the eye lands. Similarly to myopic patients, there are hyperopic patients can have a different sized eyeball. In comparison to an emmetropic eye, a hyperopic eye is too short. Basically, these eyes stopped growing a little too early. When light enters the eye, it will land behind the retina creating that blur at distance and near. To correct this, patients will be prescribed plus lenses to converge the light entering the eye and bring it forward to land on the retina. There are also patients that have the ideal eye length like an emmetropic eye, but they just have too much power in their eyes. Again, to correct this, optometrists will prescribe plus lenses to cancel out the extra minus power in the eye.