Imagine the sidewalk as the "film" in a camera -- capturing the scene in front of the pinhole apparatus. It looks like a double exposure but is in fact a single image as it existed inside the camera obscura.
I've been drawn to alternative photographic processes for many years. Additionally, I'm fascinated by the tiny, overlooked details that are literally beneath our feet as we move through the world (see my "Cement" series, elsewhere in this section of the website). Embarrassingly, when I visit grand "scenic overlooks" I'm often found on my hands and knees, examining (or photographing) some scrubby little weed, or rock. A pinhole camera, that projects the scene in front of it down onto the ground beneath (and inside of) it, where it can be photographed with a high-resolution digital camera, is a perfect combination for me!
It's not a double exposure -- the surface under the camera is being photographed with the projected image overlaid on top of it. What you see in these images is exactly what you would see if you could be inside the box. The projected image interacts with the ground in composition, luminosity, color, and texture, in ways that cannot be replicated via digital manipulation.
Composing photographs is thus a matter of visualizing how the physical elements of the ground underneath the camera will interact with the view in front of it. There is no way of "previewing" the image short of going through the full process of making a photo, which can take fifteen or twenty minutes of setup and exposure time. When the substrate beneath the camera is relevant in some way to the image being projected onto it, I've struck gold. It doesn't happen often!
The technique is not rocket science, but overall it's quite difficult to master. Light leaks are a constant problem, and with exposures of up to ten minutes the learning curve is similar to working with any large format film camera. The cumbersome nature of the process appeals to me, because it forces me to really think through the photo before committing the time to creating it.
Technical Notes: The pinhole is 1mm diameter, and the focal length is nearly 25" -- which means there's almost no light entering the camera. Effectively, I'm shooting at f /635! Depth of Field is, practically speaking, infinite. Paradoxically, though, because the image that I'm capturing with the digital camera is nearly flat (e.g. the sidewalk, with pebbles or dirt etc.) I am able to shoot at extremely wide apertures, typically f /1.4. That, along with an ISO of 800 (sometimes higher) gets me exposure times that are practical while retaining good image quality.
