I don't take you as "combative".
There are a number of factors that affect the focal plane of a lens... glasses with different refractive indices are used with different curves, both spherical and aspherical to converge all light rays onto the same plane ... as perfectly as possible (within reason, in any event). The image seen at "full aperture" is a collection of many ray traces, some in good places - others not. We see, and the film is affected by, an average, or root-mean-square ... or something like that ... of all of them.
Limiting the ray traces to those in the center of the lens CAN - and usually is, to a point - a "good thing". Depending on the design, favorable ray traces MAY be eliminated as well, lessening the overall quality. At some aperture, diffraction, which is separate from refraction, will be the dominant factor. Rarely is the aperture affording the "best" quality, a.k.a. "sweet spot", located close to the point of "diffraction dominance," Rarely, although possible. Most of the time, the design will converge the rays best at the aperture expected to be used the most ... near "wide open" for fast lenses; near the "middle" for ordinary camera and enlarging lenses.
As a side comment ... Optical Design Engineers are, as a class, the most intelligent, best damn engineers of all. Every one I've ever met seemed to be a cut above the rest.... They have to be ... Optics is one of, if not THE most, difficult field of all.
