What is "Field Curvative"?

Field curvature means the spherical shape of the focus field. Theoratically, we would assume that the depth of field (the sharpness zone dependent on the object magnification and focus distance) starts at some focus distance x in front of an focused object and ends at some distance y behind. The difference between x and y is called the depth-of-field.

In the real life there's no perfection though and the focus field can be rather distorted (as opposed to flat) with consequences for the depth of field. For sure this isn't overly field relevant but at the image borders the focus field turns inwards (towards you). In infinity focus situation this may cause some focus blur for distant object located at the edges of the image field. It is also relevant for flat objects.



Exaggerated sample :