Why does Eiken have his camera?
They didn't mention anything like this in manga but lets just assume that he has more than one camera. At the first time Sengoku found Eiken's camera inside the plane, it was white.

First camera



And the second camera, when Sengoku met Eiken, it was black.

Second camera



So I think the one that Eiken left on the plane is the first camera, later on he only use the second camera for the rest of manga.

They also mentioned it on wiki


  Considering his first camera was blown up during the plane which he left behind, it is suggested he has more than one camera, as well as numerous batteries on hand.