Many of the conventions in photography we take for granted today can be traced back to Levitsky. Some have become so popular that we forget their source.
Czar Nicholas II, his wife Empress Alexandra Feodorovna and their daughter Olga by Sergey Levitsky (1890)
Among other things, Levitsky appears to have invented the use of multiple poses for the same model, the approach of mixing natural and artificial lighting, and the idea of interchanging backgrounds for the same shoot. He also worked on improving focus by inventing the bellows camera, which allowed the photographer to move the lens in and out according to the relative distance of the subject.
Alexander II, and his Irish Setter by Sergey Levitsky
There is something deeply poetic here. In each of these cases, we see two things coming together that had always existed but had never been used before in tandem. Bellows are a distinctive feature of accordions; attaching a lens and a camera to them was a stroke of genius. Photographers always had a background set for their studios, but Levitsky used several to pack more into a single shoot. Photographers might use artificial or natural light, depending on the setting, but Levitsky understood that together they could illuminate subjects in a way that neither one, taken individually, could do. Models would typically hold one pose and hold it (with some discomfort) until the photographer let them move again, but Levitsky realized that a series of different poses was perhaps more interesting.
Portrait of Zhedrinsky, Apukhtin, Tchaikovsky, Kartsov by Sergey Levitsky (1884)
There is also a prosaic explanation to all of this. From a practical point of view, he clearly developed these techniques as a way to achieve improved results. But the more interesting consideration is that approached invention as the act of combining things, just as a poet puts two words together that were always there, but that, when paired for the first time, produce a shock and make us look differently at the world.
Alexander II in his study room by Sergey Levitsky.