What is double exposure in photography?

Secondary exposure is an opportunity shooting technique. You can shoot with a 135mm camera, which is easy to master. All you have to do is shoot the whole film, and don't bother to remember what you shot. Then, rewind the tape and still take another picture with this film. Black-and-white and color films can be used for shooting. The only thing to note is that the picture taken for the second time must completely coincide with the image taken for the first time. Since the film will be exposed twice, don't expose it all at once, but reduce the exposure by half. As far as I know, Canon's digital SLR can't be exposed twice. Nikon will do, or at least the D90 will do. In addition, PS can also be used for secondary exposure, adjusting the transparency of pictures and superimposing multiple pictures.