The Copying of an Idea vs. Expression
Copyright law protects only the expression of the idea, not the idea itself. Specifically, 17 USC Section 102 states that:
Copyright protection subsists, in accordance with this title, in original works of authorship fixed in any tangible medium of expression, now known or later developed, from which they can be perceived, reproduced, or otherwise communicated, either directly or with the aid of a machine or device. . . .
In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
In sum, a photographer can have copyright protection for a particular expression of an idea. If another photographer expresses the same idea too much in the same way as the first photo, it may constitute an infringement.