1. An image processing apparatus comprising: a restrictive condition storage unit storing restrictive conditions to be applied to an image to be output; a processor in communication with the restrictive condition storage unit, the processor configured to: accept an image that is obtained by shooting a subject and has at least one field; acquire at least one restrictive condition from the accepted image by: acquiring values of all pixels of the at least one field; performing regression analysis with respect to the values; obtaining a regression surface based on the regression analysis; determining an upper and lower threshold from the regression surface as the at least one restrictive condition; instruct the restrictive condition storage unit to store the at least one restrictive condition; change the at least one field based on the at least one restrictive condition so that the values of all pixels of the at least one field fall within a range between the upper and lower threshold; and output the at least one field, wherein at least two pairs of at least one restrictive condition and a feature amount condition, which relates to a feature amount of an object in the field, are stored in the restrictive condition storage unit, and wherein the processor is configured to: acquire at least one predetermined feature amount from the at east one field of the accepted image; determine at least one feature amount condition that matches the at least one feature amount; acquire, from the restrictive condition storage unit, at least one restrictive condition that constitutes a pair with each of the at least one feature amount condition; apply the at least one restrictive condition to the at least one field of the accepted image; and change the at least one field so that the at least one field satisfies the at least one restrictive condition, and acquire at least one new field.