June 10th, 2010
Why do the wagon wheels in westerns usually look like they are rolling backwards?
This is called the strobe effect (strobos is a Greek word meaning ‘the act of whirling’). You probably know that films are a series of individual pictures, or cells, that are run together giving the illusion of movement. Each picture is framed by a border that keeps it separate from the picture taken before and after it. This is where it gets the name cell, a small enclosed area. (To get an idea of their appearance, just remember what the old camera negatives looked like).
Basically, it’s to do with speed of film versus speed of wheel turning. The film used to make the westerns ran at a rate of 24 frames per second. That means 24 separate cells pass through the projector every second. Our eyes detect the 24 little snapshots per second. This flashing of images is processed by the brain and rather than observing each individual shot, we see them running together as movement.
Cameras use shutters to let light in, allowing a photograph to be taken. Shutter speeds are very fast, so as not to overexpose the film. In the old films, getting shutter speeds and frame rates to move in synch was difficult. Add to that, the impossibility of timing the wheels to turn at the same speed as everything else, and you can see how there is room for imperfection in what we see as we watch it. If the wagon wheels being filmed are rolling slower than the film on the camera is capturing their movement, they appear to roll backwards. As the wagon increases its speed, the spokes of the wagon wheels can then appear not to move at all! Quicker still and the spokes look like they are rolling faster than the wheel frame. That one is a rather strange effect.
So, simply put, if the wheel is rolling backwards, the film is moving faster than the wheel. If it is rolling relatively in synch then wheel and film are going at about the same speed. When the wheel is apparently moving too fast for the rest of the wagon and wheel frame, it is traveling faster than the film. This effect is seen on other devices too. The next time you watch a roulette game, take note of the direction the wheel is spun versus the direction our eyes see it move. What’s moving faster in that instance, the film or the roulette wheel?
Did You Know?
In Clint Eastwood’s movie, Outlaw Josey Wales, Bob Hoy features as one of the Texas rangers on the hunt for him? Watch out for him next time you see the movie.