1. An image acquisition apparatus comprising: an image acquisition unit which includes an image acquisition device and is 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 are combined; a noise calculating unit configured to calculate an amount of noise or an S/N ratio for the case in which the plurality of preliminary image signals are 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 based on the dynamic range and the amount of noise or the S/N ratio; wherein the received-light-level-difference calculating unit includes: an approximate-curve generating unit configured to generate a first approximate curve of the dynamic range versus the received-light-level difference and a second approximate curve of the S/N ratio versus the received-light-level difference; and an intersection calculating unit configured to calculate an intersection of the first approximate curve and the second approximate curve; and wherein the received-light-level-difference calculating unit calculates the received-light-level difference at the intersection as the received-light-level difference of the image acquisition device among the plurality of actual image signals acquired during the actual image acquisition.