This article offers a novel perspective on the specific understanding of the photographic sign, situating the trace at the core of its functioning. The photographic sign is subjected to Peirce semiotics analysis, which reveals its underlying structure. This decomposition allows the establishment of a semiotic model that clarifies the essence and characteristics of photography and draws certain broader lessons about the notion of trace. On this basis, three categories of object are distinguished the trace, the indicant and the print. These three objects constitute what we call the Trichotomy of the trace which takes on its full meaning in the forensic field. Building upon this triadic model and our analysis of photography, we propose a refined definition of the trace.