Editing Optical aberration (section)

===Curvature of the field of the image===
{{Main|Petzval field curvature}}
If the above errors be eliminated, the two astigmatic surfaces united, and a sharp image obtained with a wide aperture—there remains the necessity to correct the curvature of the image surface, especially when the image is to be received upon a plane surface, e.g. in photography.  In most cases the surface is concave towards the system.<ref name=EB1911/>