An imaging apparatus comprising: an optical system which is variable in focal distance; a data storage unit configured to store data indicating a correspondence between a value of the focal distance and a value of a size of an image circle, the image circle being an effective image formation region of an optical image of a subject; an imaging unit configured to obtain image data from the optical image formed on an image pickup device via the optical system; and an image processing unit configured to perform shading correction processing that differs between image data corresponding to an inside of the image circle and image data corresponding to an outside of the image circle among image data obtained in the imaging unit on the basis of the data indicating the correspondence, wherein the image processing unit is configured to perform the shading correction processing using image data in a single frame as the shading correction processing for the image data corresponding to the inside of the image circle, and perform composition processing of image data in multiple frames as the shading correction processing for the image data corresponding to the outside of the image circle, characterized in that the optical system is configured to vary the size of the image circle by the focal distance; and the image processing unit is configured to search a search region in the image data for a comparative pixel block which is highly similar to a target pixel block around a target pixel in the image data, perform, processing of a weighting addition of the target pixel and a comparative pixel in a center of the comparative pixel block that has been searched for, and vary a size of the search region and a weighting factor for the weighting addition depending on whether the target pixel is a pixel inside the image circle or a pixel outside the image circle. an optical system which is variable in focal distance; a data storage unit configured to store data indicating a correspondence between a value of the focal distance and a value of a size of an image circle, the image circle being an effective image formation region of an optical image of a subject; an imaging unit configured to obtain image data from the optical image formed on an image pickup device via the optical system; and an image processing unit configured to perform shading correction processing that differs between image data corresponding to an inside of the image circle and image data corresponding to an outside of the image circle among image data obtained in the imaging unit on the basis of the data indicating the correspondence, wherein the image processing unit is configured to perform the shading correction processing using image data in a single frame as the shading correction processing for the image data corresponding to the inside of the image circle, and perform composition processing of image data in multiple frames as the shading correction processing for the image data corresponding to the outside of the image circle, characterized in that the optical system is configured to vary the size of the image circle by the focal distance; and the image processing unit is configured to search a search region in the image data for a comparative pixel block which is highly similar to a target pixel block around a target pixel in the image data, perform, processing of a weighting addition of the target pixel and a comparative pixel in a center of the comparative pixel block that has been searched for, and vary a size of the search region and a weighting factor for the weighting addition depending on whether the target pixel is a pixel inside the image circle or a pixel outside the image circle.