An observer who sees one small point of light with one eye and a second small point of light (vertically displaced from the first) with the other is able to align the two spots with reasonable precision while reading a fine line of letters on a chart which is visible to one (and only one) of the two eyes.1 The angular deviation of the lines from the centers of the observer's entrance pupils to the two lights from a horizontally parallel position at the time of alignment can be taken as a measure of the lateral phoria position. This position is influenced by a number of different stimulus conditions, including the accommodative state of the observer, the size of the test chart, and the distance from the observer to the test chart.
The variation in the magnitude of the phoria measurement as the accommodative state of the observer changes
ALPERN M. Phoria Testing Distance Effect on Measurement at Various Accommodation Levels. AMA Arch Ophthalmol. 1955;54(6):906-915. doi:10.1001/archopht.1955.00930020912014