What is Curvature of field?

Curvature of field is also known as "image field curvature". When the lens has field curvature, the intersection of the entire beam does not coincide with the ideal image point. Although a clear image point can be obtained at each specific point, the entire image plane is a curved surface. In this way, the entire image plane cannot be seen clearly during the microscopic examination, which makes observation and photography difficult.