Question on Field Flatness

Is flatness of field in a doublet or triplet determined solely by the focal length? I had the impression that was so, but keep seeing owners referring to how much flatter the field is in one refractor vs. another. Obviously, my statement wouldn't apply to more complex designs with integrated field flatteners, and I understand that longer focal length scopes have flatter fields. Just curious. Is there much variation in field curvature for refractors of a given focal length?

- Jared Willson

>>Is there much variation in field curvature for refractors of a given focal length?>>

No. The field curvature is approximately 1/3 of the focal length. However, the effects of field curvature are aperture dependent. If one 1000mm scope is 100mm aperture and another is 130mm, then the 100mm aperture will show less off-axis aberration.