
Two of the Seidel aberrations are closely related, namely field curvature and astigmatism.
Field curvature is where the best focus of the image lies not on a plane but on a curved surface.
Astigmatism is where the tangential rays and sagittal rays do not focus at the same point. The tangential rays focus on a curved surface and the sagittal rays focus on a different curved surface.
There is an equation that provides a relationship between the radii of these three surfaces. I don't recall what the formula is, but it can be found on line. However, as I recall, one consequence of the formula is that it is impossible to correct for astigmatism unless one also corrects for field curvature.
There is also a theorem that relates field curvature to the refractive indexes and curvatures of the lens surfaces.
One other thing. Astigmatism contributes to the MTF curves, as do the other aberrations. As a result, the MTF curves in two directions are not equivalent, so one needs to MTF curves.