Simply put, multiple exposure compositing is a powerful tool in the digital photographer's arsenal, and it's fairly simple to do. Unfortunately, it's also fairly simple to screw up, if you're not careful. So, this week and next I'll offer my favorite tips for better results when compositing multiple exposures. Next week we'll cover how to successfully composite them in the computer, but for now we'll start with the most important first steps: how to capture those exposures more effectively in camera.
1. The key when it comes to creating multiple exposures to be composited together is to keep the camera from moving—and I mean moving the littlest bit at all. A tripod is, quite obviously, key. But, so is taking extra precautions to make sure the tripod doesn't move an inch—like sandbagging the legs for stability, or staking the feet to the ground or taping them to the floor for added security.
2. Accidentally kicking the tripod isn't the only way to create misaligned exposures. The subtle movement that occurs when you touch the camera to press the shutter release can be enough to make the images fall out of registration. So, if it's at all possible, do whatever you can to keep your hands off the camera. A cable release or computer-based tethered capture is a great way to trip the shutter without having to touch the camera at all. Failing that, at least use a short shutter timer to get your hands off the body during the moment of exposure, and tread lightly—very lightly!—when it comes to touching the camera to trip the shutter or make adjustments to the exposure. The more you touch the camera, the more likely you're going to have more alignment work once you get your exposures into the computer.
3. One of the most common reasons I find myself shooting multiple exposures for compositing is so that I can selectively add light to various parts of an image. Maybe I want to create a highlight on a too-dark element in an architectural interior, for instance, or perhaps I want to create a light painting effect by selectively illuminating several different elements in a scene. Whatever the purpose, I find that a handheld flash can be the perfect tool for the job. I can gel it orange to match tungsten ambience, or add a softbox or umbrella, or just bounce the flash off the ceiling for overall softness. Likewise, I can add a snoot or grid spot to put a selective spot of light on a particular area in the shot. As long as I'm careful not to point the light toward the lens, and to keep my body out of the way of the area I'm illuminating as I move throughout the scene, I'll end up with separate exposures that can be layered together neatly to incorporate the best parts of several shots.
4. When you're shooting multiple exposures to make changes to individual areas of a scene (adding bits of light here and there via flash, for instance, or moving or removing individual elements in the scene) maintaining exposure consistency is important. What you want is for every pixel to remain identical from exposure to exposure—except for those select few pixels you're deliberately trying to change. Just as kicking the tripod would change those pixels by throwing the shots out of alignment, so would changing the exposure make it harder to blend those two exposures together. The caveat here is, of course, if it's deliberate exposure changes are part of the plan—if you're making high-dynamic range images, for instance.
5. When I'm not specifically making multiple exposures in order to add or remove particular scene elements, I still like to make additional exposures after removing specific objects from the shot. For instance, on an architectural interior I might remove a lamp from a side table or a computer from a desktop simply so that should I decide I don't like those elements later I can easily accomplish the removal without having to reconstruct the missing area of the scene. Instead, I'll now have a shot of the blank wall behind the lamp that's ready to go at the click of a button. I learned this technique after working with a particularly fickle client who—even though she was on set during the shoot—would invariable call me up days later to say something to the effect of, "We love it. But, can we see it without the lamp?" If I'd made that lampless exposure, I could confidently accomplish her wildest request. If not, I was in a world of hurt. It's a technique I still use today, just as a matter of course. I was recently shooting an interior of a hotel room, and decided to remove a throw pillow from the bed, and the bench at the foot of the bed, after the shoot was complete—just in case. Not only did this allow me to add more light to make the footboard look better in one exposure, I had the option of removing those elements from the finished scene should I (or my client) make that choice later.
Tune in next week for part two of these compositing tips when I'll cover how to make the most of multiple exposures once you get them into the computer.