Putting Things on Top of Other Things: Strategies for Image Stacking
In a single shot, cameras can't come close to matching the image making potential of human vision. But by combining more than one shot, we can get a good deal closer.
Back in the days of film, it was said that taking a photograph with a camera represented merely the first half of photography. Once the film was exposed, the second half could begin with the development of that film in the darkroom. Common digital techniques including dodging and burning, and the puzzlingly named method of sharpening known as the unsharp mask have their origins in the world of film and the chemical darkroom.
In-camera double exposure techniques still sometimes employed with digital cameras also began with film. Photographers would shoot an image on a frame of film, often masking a portion of the frame to restrict the exposure to just the intended area. They would then rewind the film back to that same frame and shoot a second image, perhaps masking the opposite side of the frame. Through a planned sequence of steps, a composite image was built up in the camera – and image that never actually was but might have been, perhaps in some alternate reality. Superimposing a gigantic sun or moon in an otherwise normal image was a common strategy for effect.
Film photography wizards not content merely to press the shutter release also pioneered multi-shot techniques for superimposing one image atop another during development and printing in the darkroom. Some creative artists went quite far down this path, compositing images of leprechauns living in tree trunks and other fantasies. This took a great deal of time and skill, and while the results might have been fun to look at, there was almost always tell-tale fringing and other artifacts evident. The techniques were just too crude to avoid detection as forgeries.
But digital photography has changed things. The chemical darkroom has yielded to the digital darkroom, and all sorts of things are possible now. With enough time and the right software, multiple shots can be combined with even more precision than previously possible. With such power comes great responsibility. Yes, photographers can still paste a gigantic moon over a shot of the downtown skyline, and yes, photographic evidence of leprechauns frolicking in the forest can still be manufactured, but there are also more practical uses for combining multiple shots digitally.
Yet cameras still aren't perfect. As I said at the outset, cameras have limitations when compared with human vision. The sensor involved in each is only part of what differs between them. Arguably more significant is the fact that what we "see" is only partly determined by what our eyes relay to our brains. Once the signal gets there, the real magic starts. We routinely believe we are seeing what in fact is a composite of multiple images – multiple sights. We look up and our eyes focus on the distant mountain peak. Our pupils contract to compensate for the fact that its snowy slopes would otherwise be blinding white. We look down at the ground and the focus of our eyes adjusts accordingly. Our pupils dilate to allow us to register detail in the shadows of the forest floor. We believe we are seeing both at the same time, but we are actually combining multiple impressions into a single understanding of the world in front of us. And all this without our conscious effort. Perhaps this isn't really magical, but it is indeed quite extraordinary when compared to what cameras do.
Digital darkroom software today though can allow us to simulate this magic by combining multiple discreet camera photographs shot with different settings into a single composite image. All sorts of things are possible, but here are some of the digital darkroom image stacking strategies that have proven most useful in practice.
Exposure stacking allows the creation of images that exceed the subject brightness range possible in a single shot. The latitude that newer sensors can record without becoming overwhelmed by noise has improved considerably, but brightness in nature varies even more widely. Expose a single image for the sky and the foreground will fall into near total darkness. Expose a shot for the foreground and the sky will be rendered as burned out white or close to it. But combine those two images together in the digital darkroom and you can create a composite that overcomes the limitations of each. The results can look perfectly normal because we are used to seeing foreground and background properly exposed. The contraction and dilation of our pupils happens completely automatically.
Focus stacking allows the creation of images with depth of field not possible in any single image. Even with the lens stopped down to the smallest possible aperture, no one shot can encompass the depth of field that exists in every situation. But just as our eyes automatically refocus as we glance this way and that, and just as our brain automatically combine those glances into a composite whole, focus stacking lets the photographer mimic this same strategy. By shoot a series of images focused at increasing distances and then combining them later in the digital darkroom, you can create composite images that look perfectly natural because we are used to seeing things that way, even though a camera and lens can't achieve such results in any single frame.
But these aren't the only image stacking strategies possible. Getting comfortable and watching the sun set can be an awe-inspiring experience that can be difficult to do justice to in a single photograph. The best color in the sky will likely happen close to or at twilight, a time when the foreground landscape will be almost fully cast in shadows. While it is possible to render detail in that foreground via exposure stacking, the results may not be fully satisfactory. Mentally, we remember the colors of the foreground from before the sun went down and combine that experience with the colors of the twilight sky. And there's no reason we can't do a similar thing with a camera. Shot one frame with the sun still above the horizon. To avoid excessive glare, use a lens hood and perhaps even shoot early enough that the sun isn't even yet in the frame. With your camera firmly mounted on a tripod, relax and wait for peak color in the sky, then shoot again. These images can then be digitally stacked and blended later to create a composite that looks the way your mind thinks it's supposed to, even if the source material was shot over a period of half an hour or more. Once you start thinking about things, the possibilities are limited only by your imagination and creativity.
What we want are images that look the way our mind tells us they are supposed to look – the way we think things actually do look, even when such vistas may not ever actually exist at any one time and without our eyes shifting focus or dilating to adjust for exposure. There's no reason we can't use image stacking in digital darkroom to come as close as we can.