Standard clinical studies of the visual fields provide for determinations of the peripheral field on the perimeter and central fields on the tangent screen. On the perimeter arc, useful clinical isopters have been worked out for as small a visual angle as 1/330 white. These were carefully described by Ferree and Rand1 in 1931, and formed the basis for the Montefiore charts described by me2 in 1948. The 1/330 white field, which is the innermost peripheral isopter studied by Ferree and Rand, extends to 64 degrees temporally and 47 degrees nasally. It may well be that in many cases the nasal field does go out as far as 50 degrees and the temporal to even a little better than 65 degrees. As for the tangent screen, isopters can be measured only up to 25 degrees because of the increasing tangential differences producing inaccuracies beyond this limit.
CHAMLIN M. An Isopter in the Intermediate Field of Vision. AMA Arch Ophthalmol. 1959;61(4):608-615. doi:10.1001/archopht.1959.00940090610016