The focusing power of the eye is too strong and a negative (-) lens is required to correct vision. Glasses are required to provide good long distance vision.
The focusing power of the eye is too weak and a positive (+) lens is required to correct vision. Your vision is clearer at distance than your close-up vision.
Astigmatism causes blurring at all distances. The eye is not perfectly round; it’s more like a rugby ball and its surface is not symmetrical. It focuses light from one direction better than from others.
A cataract is when the lens inside the eye becomes cloudy. As a result the lens interferes with vision which becomes blurry or darkened. Another symptom of cataract is ‘glare’. If untreated, vision becomes progressively worse. This condition usually affects people after the age of 60 to 70 but could occur earlier.
Presbyopia is a gradual process starting at about 45 years of age when people find it increasingly difficult to see objects close up. This is caused when the lens inside the eye becomes less flexible. Presbyopia makes it hard to change focus from near to far and back again. If you have short-sight and are over 45, you may take off your glasses to read, or use bifocal glasses to see at both short and long distances.