What constitutes having poor vision?

Lowered acuity: Poor vision is vision less than you would like. There are legal standards for instance in most states 20/200 is the limit for legal blindness. Poor vision can also be from restriction of the field of vision (peripheral vision). Poor vision is indicated by the best you can do with the best correction.

Less than 20/20: Using what is called a snellen acuity, both ophthalmologists and optometrists define normal vision as 20/20 in the us. This means that you can see the small letters on the eye chart at 20 feet equivalent. Less than 20/20, such as 20/40 means that you see at twenty feet what a perfectly sighted person could see at forty feet. The british use a 6 meter system so 6/6 is perfect vision.

Decreased contrast: Decrease in contrast sensitivity (CS) with conditions like macular degeneration, diabetic retinopathy or even previous optic neuritis can also play a role in poor vision. A patient could have 20/20 vision but poor contrast and have visual symptoms.