Well, you can! While double exposures are fairly unpredictable, sometimes frustrating, and always fascinating, an intentional double exposure (shooting multiple exposures on the same frame) is neither magic nor rocket science. It's an adventure in which the photographer is often rewarded with unexpected and interesting results. It's like going boldly where none have gone before!

Multi-Exposure Technique
The multiple exposure shooting technique is straightforward enough, shooting two or more images on one frame of film to create a new, combined image. Rather than capturing a single distinct image (a "decisive moment" as Cartier-Bresson would say), we get something with double and multiple exposures more akin to a collage, where disparate and unrelated things overlap and interact, where time is stretched and warped between events. It's creating a still shot, but with almost cinematic overtones.

Double exposure is like magic, and therefore, a bit tricky. There are a few dicey areas, but, as any seasoned magician will tell you, the secret to success is in good timing. And time (and I like a good time as much as anyone) remains one of the two main ingredients, along with aperture, to getting a good exposure. The problem here, with shooting multiple images on the same film frame, is that the cumulative exposure in overlapping areas (particularly in the highlights) can result in overexposure and loss of image detail. That is, unless a proper exposure strategy is employed.

In order to keep the combined light collected from two exposures from overexposing the film, the photographer needs to reduce the exposure of both images by either rating the film's ISO double the normal value (for example, rating a 100 speed film at 200), or using the exposure compensation controls to underexpose each shot one stop so that the combined double exposure is consistent with the recommended film sensitivity ratings. It's not so difficult, as long as you want each of the two image components to have equal weight.

Image Emergence
However, there will be many cases where you want one image to have predominance over the other, in effect to be the focus of the picture, with the secondary image acting as a background. In that case, it is better to favor the main image on which you wish to concentrate by underexposing only 1/2 stop, while giving the background scene 11/2 stops less light. (For example, to make the picture of the mannequin's face stand out clearly from the cathedral background in a double exposure, give the face closer to normal exposure while underrating the cathedral a stop more to keep the background from bleeding through.)

Choose a good lighting situation where a well-lit subject is framed before a darker background, and it will stand out well when double exposed with a flat-lit background image, and it won't need as much exposure compensation. On the other hand, lots of bright sky will bleach out everything else it's blended with, no matter how much you compensate. With a little experimentation (and you can try bracketing combinations to test this), you will find what works best for you.

Previsualization
Here's where the creative part comes in, and where the familiar concept of "previsualization" works wonders for you. Before beginning your combination shot, decide if one image should ride above the other and then expose it accordingly by giving it more exposure. But don't forget that the total exposure must still fall within the normal film speed guidelines for a correctly exposed single exposure! Once this concept becomes second nature to you, you're ready for some fun.

Generally, I'll shoot my foreground image first, and then go looking for a context shot to complement it. A blend of sharp image and soft texture, of geometric and organic forms, or stillness and frenzied motion, can result in some very interesting and exciting visual collages. Of course, rules are made to be broken, and in photography some of the best and most striking pictures happen this way. Keep yourself open to the possibilities, and maybe the accidental masterpiece will reveal itself.

Camera Technique
Many of today's cameras incorporate multi-exposure functions allowing for proper alignment, precluding the need on older cameras to disable the advance mechanism. (If you're still shooting one of these antiques, first you need to tighten the film advance lever until it's taut, then depress the rewind release button on the camera bottom, and cock the film winder while holding this button down.) As for exposure, you can, of course, choose to overate your film by one stop for double exposures, and shoot either in speed or aperture priority, or totally manual. I prefer the flexibility of shooting in aperture priority mode while using the exposure compensation dial (like on my Canon EOS 2A and featured on most recent cameras), and adjusting my exposure ratio according to the lighting/subject circumstances.

It's entirely up to you, and that's a small miracle in this day of computerized automation! Don't forget, you are not just limited to double exposing. With the modern multi-exposure devices you can take triple and multiple exposures to your heart's content, without worrying about frame registration. But remember that you must compensate accordingly, because the density of the image builds with each additional exposure, and that film's ISO is fixed and unforgiving.

Personally, I have been experimenting with in camera double exposures for years, not because I couldn't remember to wind the film, but because it was closer to my "normal" way of seeing things. Due to an eye muscle problem, I had been plagued since childhood with double vision. I saw two of everything! As a youth playing baseball, I remember being beamed too many times trying to catch fly balls, or swinging at the wrong ball while batting. I was nearly always last to be picked for a team.

I guess either I'm a slow learner or a masochist, but one day, after 40 odd years, I got brave and finally decided to undergo corrective surgery. Then suddenly, I realized that if this operation were successful, I was going to lose my way of seeing. Two weeks before I was to go under the knife, I rushed out to preserve this endangered double vision while it was still "native" to me. I went on a mission to New York City to shoot nothing but double images, as a memorial to my multiplex visual perception. The Big Apple was the perfect subject, and the resulting harvest of images became sort of a visual collage to my future past. In those double images I documented that sense of optical bombardment and simultaneity of visual stimulation one experiences in "the city that never sleeps." Afterward, I was given a solo exhibition of this work at the Kodak Photo Salon in Tokyo. It was titled, "New York Energy."

Today, when I'm shooting a camera, unlike baseball, I no longer have to worry about which is the real target and which is the ghost image, and my double vision days are history now. But I still recall how things looked in multiplex, and can't help but wonder if that isn't the way our minds store images, like a visual stew, all mixed together. And once in a while, when I get tired of another properly exposed and predictable "perfect" image, I'll go out and bang off a few double exposures, just for old time's sake.