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My understanding is that a newtonian and most other scopes inherently have a curved focal plane. Im not sure if thats called positive or negative curvature.
My other understanding is that some of the longer fl pentaxs have a focal plane curved in the opposite manner, exacerbating this field curvature issue.
If the scope is one designed with a flat field, the issue would be less noticeable.
FWIW, in my 14mm pentax its noticeable if I go looking for it , but its not intrusive to me. I have seen far worse examples of this with the meade s5000 plossls being shockers for this abberation at f5.
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