As per the American Optometric Association adults who wear glasses or contact lenses and adults who are over 60 years should visit their eye care specialist every year or as suggested by the doctor. If the person doesn’t require any vision correction, then a person between 18 and 60 years should visit the doctor twice a year.

Low vision is a term describing a level of vision below normal (20/70 or worse) that cannot be corrected with conventional glasses. Low vision is not the same as blindness. People with low vision can use their sight. However, low vision may interfere with the performance of daily activities, such as reading or driving.

"20/20 vision" is commonly accepted as the standard of normal distance vision for a human being. Basically it means "good visual acuity at 20 feet." So if your vision is 20/20, you can read certain sizes of letters on a Snellen chart clearly at 20 feet or closer. But if your friend has 20/15 vision, his visual acuity is better than yours: you would have to stand 15 feet away from the chart to read the smaller letters that he can read while standing 20 feet away. Conversely, someone with 20/30 vision has worse distance vision than you.