The fields of vision are tested clinically by using a chosen working distance and by varying the size of the test object. This method is convenient and by custom has become part of the practice of ophthalmology. However, this scheme of testing, and the mathematical analysis which obviously results, leads to a fallacious understanding of the limits of visual space.
The error arises from the fact that in daily experience observers are not considering 0bjects which vary in size, but are concerned with objects which remain constant in size and vary in distance. A baseball does not grow or shrink in size; it comes nearer or goes away. The size of the visual field for a given object varies with the apparent size of the object, which is a function of the distance of the object from the observer. The mushroom-shaped island of vision so often reproduced from the
WELSMAN HS, McCULLOCH C. The Fields of Vision and Their Significance When Projected into Space. AMA Arch Ophthalmol. 1955;53(5):694-699. doi:10.1001/archopht.1955.00930010702012