Nightblindness can be an inherited visual defect, sign of an eye disease, or deficiency of Vitamin A. Vitamin A is needed to produce a pigment in the eye that allows you to see in dim light and at night. Vitamin A is found in liver, and yellow, orange and green vegetables. It is preventable if treated early enough. The test for it is that uncomfortable puff of air in your eye at the eye doctor. Treatment includes oral medication, eye drops and possibly surgery.
Prevention is regular eye exams and testing.
This is when there is too much distance between the lens and the retina to focus a clear image. There is also now a new surgery that can correct the problem. This is the type of visual problem most people have. This is when objects close up are blurry but you can see objects far away just fine. This occurs when the is too little distance between the lens and the retina.