Refractors: Curvature of field

A system which is designed to remove completely astigmatism suffers from curvature of field, as the points of sharp focus lie on a curve rather than on a plane. Flattening of the field can be achieved at the expense of having incomplete removal of astigmatism and usually some compromise is made. The judicial placement of stops in the combination of lenses can also be used to reduce the effect of field curvature.