[casstony]

Field curvature is only related to f/ratio for a particular design of telescope and when the aperture is kept constant. It's more correctly related to the focal lenth and radius of curvature of a newtonian main mirror, such that a very large aperture, fast mirror will have less curvature than a small aperture, slow mirror.(the large mirror having a longer focal length than the small mirror).

Different designs and sizes of telescope have varying degrees of field curvature and eyepieces are designed with different amounts of curvature. How the telescope and eyepiece interact determines whether or not curvature is visible at the eyepiece. If the respective curvatures are of the opposite sign they tend to cancel each other out; I think this is why the Andrews 30mm Ultra Wide performs better in a SCT than a Newt.