ok. i've done a double exposure accidentally once. how do i do one correctly to say do a moon shot? do i have to rewind the film after a finish a whole roll? how do i know exactly what frome to go to. (Speaking SLR camera of course). any help wpould be appreciated.
The answer is going to be different depending on whether your camera is a manual wind or auto winder.
For Manual Winders
Take your first shot. Then press the film rewind release button and hold it while cocking the film advance lever. This will reset the shutter without advancing the film. Release the film advance button and re-cock the film advance lever (it won't go all the way) to make sure the film is tight. Now you're set to take your second shot. The only thing with manual winders is that the film may not stay exactly in register.
For Auto Winders
Most auto winders these days have a multiple exposure function. If yours does then you simply set it for the number of exposures you want on the one frame of film and the camera won't advance the film till that number of exposures have been taken. If it doesn't have a ME function then it needs to be able to do a mid-roll rewind. Most that I know of allow you to press a button and as long as you keep it pressed the film will rewind. So what you need to do is press that button and release it quickly so that the film only rewinds 1 frame. This is tricky to be sure and will take extensive practice to get it right. Someone else may have another idea of how to do it without the ME function. If your camera doesn't allow for mid-roll rewind then I'm not sure it could be done.
Other Things to Consider
Exposure. When you're exposing a frame of film more than once you need to adjust exposure. For a double exposure, you should halve the exposure of each frame from what you normally would. This applies most specifically for images that are going to be in register. You don't need to underexpose as much for images that will not be in register (i.e., inserting a moon in a city skyline scene - in this case you would expose normally for the moon and then expose normally for the skyline, maybe underexpose each marginally).
It is possible to shoot a whole roll of moons and then re-expose the film: yes registration might be off. The thing to do is to put the film in the same way as you did the first time(relative to the leader length exposed on the take-up). This is probably easier with an auto wind camera, as you just put the tip of the leader to the red mark. with manual wind put a small mark on the leader end closest to the cassette and wind on manually only to that point.
one thing I would add to Beachesto's directions on manual cameras is that you may need also to hold the rewind knob in place,because the clutch may be tight and require that back pressure so film doesn't advance anyway.Thats the way my minolta x-700 was. also take up the slack in the spool before your first shot. mik