Terminology
Article Index
Start | minus/plus | spherical | cylinder
myopia | hyperopia | presbyopia
look-under | look-over | plano
Prescriptions

Myopia (near-sightedness):

The eye is too long/the eye lens is too curved (too much correction). An image is focused in front of the rectina.  To correct for this a minus (-) lens is needed.

 

Hyperopia (far-sightedness):

The eye is too short / the eye lens is not curved enough (too little correction). An image is focused behind the rectina. To correct for this a plus (+) lens is needed.

 

Presbyopia (shortness of the arm :-) / reading lenses

The eye(lens) can not accommodate enough to focus on close object. This happens to all of us as we get older. Depending on the amount of decline of the power to accommodate (and some other factors) you will need reading glasses. Even more so when you're underwater. The differences of refraction between air and water makes that you'll need extra help to see close objects like your gauges. That's why even people who don't need reading glasses on land (yet) sometimes have trouble seeing at close range underwater.