The picture shows a multi-purpose flashlight made by Xiaoyang. AB is a small socket for inserting film, CD and EF are telescopic sleeves, and DF is filled with a convex lens.

Light rays parallel to the main optical axis converge at one point, that is, the focus. Therefore, according to the reversibility of the light path, if the light emitted by the filament is refracted by the convex lens, it is necessary to put the filament of the small bulb at the focus of the convex lens.

So the answer is: focus.