Here I would like to present a few technical notes on the procedures for making multiple exposures using film. For now I will leave aside any questions about style, effect, meaning, aesthetics, etc.

There is a simple rule of thumb for setting the exposure when you are exposing more than one image on the same frame of film: multiply the designated film speed by the number of exposures you wish to do and set the ISO in your camera to that. So, if you have 400-speed film and are rating it at that, and you want to do three exposures on one frame, you will set your ISO for that frame to 1250 (1200 is not available, so you must choose the nearest speed on the camera). Likewise, if you have some 100-speed film that you are rating at 100, and you intend to shoot five exposures on one frame, you will set the ISO at 500. If your camera has some multiple exposure control switch that prevents the film from being advanced automatically, you are all set. My Nikon cameras from the 1970s have such a control, and so does my electronic camera from the 1990s. One advantage of the older mechanical cameras is that you can take the various images for the multiple exposure frames at different times and places, as long as you have a way to remember how many you’ve done and how many are left. On the fully electronic cameras, or at least on my Pentax PZ-1, which rely on a battery to run, if you set it up to do some multiple exposures, you are committed to doing them in a relatively short space of time, otherwise the battery will run down. And if you turn the camera off, you will lose the multiple exposure setting, and that frame will be incorrectly exposed.

When you use this rule of thumb for the exposure setting, each image in your frame of multiple exposures will have equal weight, each one will be given the same exposure. (I’m leaving aside the entire question of planning your image taking into consideration the light and dark areas, masking, etc. — I don’t work that way.) But if you wanted to give one or even two of the images more weight than the others, there is a formula which you can use to calculate that. I will write out the formula, and take this opportunity to thank the expert on who so kindly provided it along with examples, which you can probably look up there. Here it is in all its splendour:

[A(a) + B(b) + C(c) …]/x = n where A = # of exposures at ISO a; B = # of exposures at ISO b; C = # of exposures at ISO c … you get the picture… x = total number of exposures; n = ISO rating per exposure.

I spent some time trying out various possibilities for film of different speeds and different numbers of exposures, and from a practical point of view there are not too many that work out well. Here are a few that may be useful: for 100-speed film, you can expose one image at 125 and three more at 500; for 200-speed film you can expose one image at 250 and three more at 1000, or one at 640 and four more at 1000; for 320-speed film (or film that you are rating at 320), you can expose one image at 400 and three more at 1600; for 400-speed film you can expose one image at 500 and three more at 2000; for 500-speed film (or film that you are rating at 500) you can expose one image at 640 and three more at 2500. If your math is better than mine and you notice mistakes here, please let me know.

Now, with the method described here, we are working one frame at a time, meaning that you could do double exposures for one frame, triple exposures for another, normal shooting for other frames, and so on. You can set up the ISO required for the number of exposures you intend to do for any individual frame on your roll. But another way to do multiple exposures involves the entire roll of film rather than the individual frames. This allows for a greater involvement of the element of chance and also broadens the role of the time element. The procedure is similar to that described above, and you could even employ the option of weighting the exposures differently. So, to do four equal exposures on a roll of 100-speed film for example, you would set the ISO at 400 for each pass-through of the entire roll, and you would run the whole film through the camera four times. If your camera allows you to leave the film leader out when rewinding, it makes the job much easier, otherwise you may need a film picker to retrieve the leader and pull it out each time.

With this method you can also use a cheap point-and-shoot camera that doesn’t have any manual options or controls. It probably does have an automatic DX coding reader to set the ISO, but you can hack the coding on the film canister to make it read the ISO you need. And if you don’t want to fiddle with a film picker, you could sacrifice the last frame on the roll and go into the darkroom to open the camera and rewind the film manually to leave the leader out. One potential problem with this approach is having the frames line up closely enough from one pass-through to the next. It’s best to use the same camera for each film and make a careful note of how you load the film into it, trying to duplicate that each time.

You are no doubt already dreaming about the possibilities in using this method, thinking about shooting one roll now, then leaving it aside for months or years, then shooting the second pass-through in a completely different place and season–there is definitely an enticement to the imagination here. The method also allows for a collaborative multiple exposure project, with different photographers in different parts of the world. If there are three photographers, and the first one is yours truly, then I would shoot three rolls of the same type of film and mail those to the second photographer who would shoot those same three rolls, preferably using the same model of camera, in hopes of us lining up the frames reasonably well, and then on to the third photographer who will then keep one roll and mail the other two back to me and the second photographer, so everybody ends up with one roll to keep. This would be especially important with slide film, which is my preference as the ideal viewing and presentation medium for film.