
Offense against the sine condition
The Seidel aberration S2 (coma) is closely related to the Abbe sine condition, which says that for a lens corrected for spherical aberration, coma is also corrected if

where U is the real ray angle for an object point on the axis, U' is the corresponding real ray angle at the image plane, and u and u' are paraxial angles at the object and image plane. This equation means that the magnification M=u'/u determined paraxially should persist beyond the paraxial regime as the ratio of the sines of the object and image angles.
LensForge calculates the OSC (Offense against the Sine Condition) for finite conjugate systems as

using the limiting form

for infinite conjugate systems. Afocal and input-finite-output-infinite systems are not handled by version 1.1 of the software.