What is impaired vision?
One aspect of impaired vision is the sharpness of the images you see – also known as your visual acuity. This tells us how good our eyes’ “resolution” is. That is determined by checking how far away two points have to be from one another for us to be able to see them separately from a certain distance.
People with normal vision can clearly distinguish two points that are 1.5 millimeters apart when looking at them from a distance of 5 meters (visual acuity of 1.0). That means that if they stand 5 meters away from an eye chart, they can see which way around a c-shaped symbol with a 1.5-millimeter opening is. Young people in particular often have a better visual acuity than 1.0 and can identify the symbol from even further away. The closer you have to go to identify it, the lower your visual acuity.
People are considered to have impaired vision if they have a maximum visual acuity of 0.3 even when using corrective eyeglasses or contact lenses for both eyes. That means they have to be 1.5 meters away from the eye chart to be able to see the opening in the c-shaped symbol. In everyday life, that means they might be able to read the titles in books or magazines, but not the smaller main text.
People who have a visual acuity of 0.05 or less are considered to have a severe visual impairment. Those with a visual acuity of 0.02 or less in both eyes are considered to be blind.
Other types of vision problems can be classed as a visual impairment too, though – such as a severely restricted field of vision. Your field of vision is what you can see when you look straight ahead without moving your eyes. With each eye, it is normally possible to see a total area of up to 150 degrees to the left and right, and up to 135 degrees upwards and downwards. Diseases such as a detached retina or stroke can restrict the field of vision. People with a field of vision of 5 degrees or less in all directions have a visual impairment, even if their visual acuity is normal. If people have both a narrow field of vision and poor visual acuity, even minor restrictions lead to visual impairment.