The two most commonly used subjective methods for testing astigmatism are (1) the various tests involving the use of some form of line chart and (2) those involving the use of the crossed cylinder. Sometimes, of course, both methods are combined. I am not going into the how and why of these tests except incidentally. My prime purpose is to discuss some points of fundamental differences between them. These differences are not discussed in any of the standard works on refraction, and yet they have theoretic and practical significance.
In the uncorrected astigmatic eye there are two focal lines formed for every point of the object seen. These lines correspond to the two principal meridians, are at right angles to each other and are at different distances. The space between them is the interfocal distance, better known as Sturm's interval. But the retinal images do not necessarily consist of these
JOSEPH I. PASCAL. FUNDAMENTAL DIFFERENCES BETWEEN CROSSED CYLINDER AND LINE CHART ASTIGMATIC TESTS. Arch Ophthalmol. 1940;24(4):722–730. doi:10.1001/archopht.1940.00870040108008