1. An image acquisition apparatus comprising: an image acquisition unit including an image acquisition device and configured to acquire a plurality of actual image signals by carrying out actual image acquisition of a subject and to acquire a plurality of preliminary image signals by carrying out preliminary image acquisition of the subject, before the actual image acquisition, with different amounts of light received by the image acquisition device; a dynamic-range calculating unit configured to calculate a dynamic range for a case in which the plurality of preliminary image signals is combined; a noise calculating unit configured to calculate an amount of noise or an S/N ratio for a case in which the plurality of preliminary image signals is combined; and a received-light-level-difference calculating unit configured to calculate a received-light-level difference of the image acquisition device between a plurality of actual image signals acquired during the actual image acquisition on the basis of the dynamic range, and the amount of noise or the S/N ratio.