? Two or more completely different scenes shot on one piece of film. A shot is taken, then the shutter is cocked but the film isn't advanced. Then another shot, taken somewhere else, is exposed on that same frame.

Back in the old days, when shutter cocking and film advance were separate operations, these kinds of multiples were easy (too easy, as some will attest). Most modern cameras, 35mm in particular, wind and cock the shutter in one operation (via the push of a lever or crank of a motor). So you must disengage the wind function to make multis. On some mechanical cameras, you can simply disengage the winding clutch by pushing the rewind button or lever. (Consult your manual.) On electronic cameras, however, you usually have to switch to a separate multi-exposure mode, then enter the number of exposures you want to make on a single frame.

Because each exposure adds more density to the film, you'll end up with extremely overexposed negatives or slides if you simply shoot away at the meter reading. To prevent overexposure, you must deliberately reduce each exposure. With a double image, it's easy: You just shoot the two pictures at half the normal exposure, in other words, one stop under.

But how do you figure exposure if you're shooting, say, 12 multiples? Just multiply the number of exposures times the ISO rating of your film, and reset the camera to that new rating. Example: To shoot a 12-shot multiple on 50-speed slide film, multiply 12 times 50 to get 600. Now set your camera ISO to the nearest standard rating, in this case 640, and shoot away.

? Single-subject multiples: A specialized version of the multiples described above, these are done by keeping the camera in one place (preferably on a tripod) and exposing the frame over and over again. If you shoot, say, a boat bobbing on the water, you'll get an impressionistic, blurred boat against a sharp background!

? Dark-field multiples: If you're shooting a scene with a large black or nearly black area in the frame, you can create multiple images in which individual elements don't have the semitransparent look.

A classic application of this is adding a big moon to a night landscape. A scene with a dark sky area is first shot; then, on the same piece of film, another exposure is made of the moon to add it to the sky. Unlike simple multiscene multiples, dark-field multiples don't add exposure in layers Each shot against the black back ground should be made at normal exposure.

? Flash multiples: Similar to dark-field multiples, except flash is used for the exposures. A fun shot is a double exposure of a person by a solitary tree after dark: With the camera on a tripod, one shot is made with your subject on one side of the tree; another with the subject on the other side of the tree. The two exposures of your subject look like identical twins.

You don't need multi-exposure provision to make a flash multiple - in fact, you don't even need flash synch! With a dark background, just open the shutter on B (or a long timed exposure) and fire the flash the appropriate number of times with the flash test button. Then close the shutter.

? Stroboscopic flash multiples: Some flash units (and some point-and-shoot cameras) have a setting for true Stroboscopic flash: multiple bursts in rapid succession - 10 pops a second, or even more.

Using a black background (or an open space at night), you can use strobe flash for such motion studies as analysis of a golf swing or a check of your Little Leaguer's at-bat form.