“Slit-scan”, “Photo-finish” or “linear strip” photography is a technique where an image is created while the film moves past a narrow slit. Most people are familiar with the “photo-finish” used in for example the Olympics or horse racing. Nowadays the cameras are digital but in the past, film was used. A slit-scan image is made up of many narrow slices assembled side by side along a timeline. The image is not of an instant in time as per conventional photography, but rather a timeline, where events having occurred in the same position over a given period of time are represented.
It’s possible to make slit scan images with a digital camera by shooting video and afterwards extract a single line of pixels from each frame and assemble them left to right to form the image. A script could be written to automate the process. The easiest way I know to create slit-scans is with the i-phone app , although it lacks control and only really works for slow moving objects.
A while back (2008) I decided to see if I could create a slit-scan photo on film. I used a simple pinhole camera and wound the film by hand past a 1mm wide slit that I had installed in the camera. The result while not technically brilliant was interesting. (That same photo was published in an instructional book in 2009- From Pinhole to Print )
As it worked for the pinhole camera with manual winding I wanted to see if I could get sharper and smoother results by using a lens and a motor to drive the film.
Let the insanity commence!
After a few weeks of trial and error and figuring things out I ended up with the camera shown below, an old medium format folder bodged together with a Pentax K-mount and Pentax power winder. The results were good but the problem was I only had one speed and it worked only for fast moving objects. For example I couldn’t slow it down to capture people walking by.
The latest incarnation (finished 3 years after the original) includes an electronic speed control circuit wired into the power winder. The circuit has an l.e.d. readout of the % power level and this equates to a speed of the film which I have manually measured for each setting. I can now make slit-scan images of things moving at a range of speeds. There’s one formula that governs the setting up of a slit-scan (that’s if you want to preserve scale and not compress or elongate your subjects – though that can be interesting too!).
film speed = (Focal length of lens x subject speed) /distance between camera and subject
Here’s a photo-finish I took on the 2 July 2012 at the Nantes Hippodrome, the camera can capture the entire field of runners as they cross the finish line.
Check out the large version “here“
It works well for cars too…
…and people, note how the slit-scan technique makes everyone appear to be walking in the same direction. The woman in red and the woman putting her coat on are actually walking to the right. This effect is because the image is a time-line and everything is made to appear to move in the same direction that the film is travelling.
I’m looking forward to trying it out on other sports – track and field, football, rugby, motor racing and cycling (I’m hoping to catch a stage of the Tour de France this month).