Eye examinations may detect potentially treatable blinding eye diseases, ocular manifestations of systemic disease, or signs of tumours or other anomalies of the brain. Visual acuity is the eye's ability to detect fine details and is the quantitative measure of the eye's ability to see an in-focus image at a certain distance. The standard definition of normal visual acuity (20/20 or 6/6 vision) is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc. The terms 20/20 and 6/6 are derived from standardized sized objects that can be seen by a "person of normal vision" at the specified distance. For example, if one can see the normal object that can usually be seen at 20 ft at a distance of 20 ft, then you have 20/20. If one can see at 20 ft what a normal person can see at 40 ft, then the vision is 20/40. Similarly, if one can see at 20 ft an object size that normally can be seen at 200 ft, then the vision is 20/200.