Even in the absence of astigmatism, there is a tendency of optical systems to image better on curved surfaces than on
flat planes. This effect is called field curvature and is illustrated in the figure below. In the presence of
astigmatism, this problem is compounded because there are two separate astigmatic focal surfaces that correspond to the
tangential and sagittal conjugates. Field curvature varies with the square of field angle or the square of image height.
Therefore, by reducing the field angle by one-half, it is possible to reduce the blur from field curvature to one quarter
of its original size.
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