حدة البصر
Visual acuity declines systematically up to 30° eccentricity: At 2°, acuity is half the foveal value, at 4° one-third, at 6° one-fourth etc. At 30°, it is one-sixteenth the foveal value. From thereon the decline is steeper. (Note that it would be wrong to say, the value were halved "every" 2°, as said in some textbooks or in previous versions of this article.)