Not isoplanatic??

Please help.
When I'm running diffraction MTF, it also appear follows message.
Warning: This system is not isoplanatic at field(s) 10.
               Any MTF results are of questionable value for assessing
               the image quality, except in those cases where the user is
               interested only in the image of a point-like object.  The
               image quality for the unacceptable fields varies significan-
               tly over a lateral region of 12 Airy disk diameters.
Can anybody help?