Q: While watching Western movies, for example, I’ve always been fascinated that the wheels on stagecoaches, for example, appear to be going backward even when the stagecoach is moving forward. I guess this has something to do with the speed of the film, so what film speed would you need to eliminate this optical illusion?
C.L., of Cahokia
A: Ever since the days when prairie schooners lumbered west across movie screens, observant viewers have been scratching their heads over this seeming contradiction: They know those pioneers are headed toward Californy, but the film makes it looks like they’re rolling back toward the East Coast.
It’s an optical illusion known in scientific circles as the stroboscopic effect or temporal aliasing. But, for your sake, I’ll use an everyday term that even John Wayne would appreciate: the wagon-wheel effect. Of course, that makes it sound a little simpler than it is to explain, but I'll try:
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Movies and TV give the illusion of the motion you’d see if you were watching cars on a street corner, but it’s different. The picture you see on a movie screen is divided into a certain number of frames per second, and although the time lapse between frames is minuscule, it’s sometimes enough to trick your mind into thinking it’s seeing something that really isn’t happening.
Let’s take a wheel with four spokes and think about it like a clock face. Now, let’s assume that the first frame shows one spoke at the 12 o’clock position. By the time the next frame is taken, that spoke may have moved to the 11 o’clock spot. You know the wheel would have had to have moved clockwise if the car was moving forward, but the eye-brain connection interprets it as moving backward from 12 to 11. If the car in the film continues at that constant speed, succeeding frames will trick your brain into perceiving a smooth — but backward — motion.
You can experience the same effect by spinning a wheel in front of a strobe light because the rapid on-off pulsing of the light will mimic the frames of a film. Given the right speed, the wheel may look like it’s rotating normally or even standing still. That’s why under certain lighting conditions it may make certain machines dangerous to operate because they may appear to be stopped or moving too slowly. Workers fooled by this illusion may be seriously injured if they try to fix the “problem” without thinking.
In the end, then, there is no one answer to your question. As the speed of the stagecoach changes, it requires a change in film speed to ensure that the image on the screen always appeared to be moving forward, thereby preventing the wagon-wheel effect. And without using computer magic, that’s probably not something even Spielberg or Lucas would want to mess with.
Where was that Yellow Rose of Texas first found growing?
Answer to Monday’s trivia: Upon witnessing the rollout of the B-17 with its multiple machine guns on July 28, 1935, Seattle Times writer Richard Williams reportedly remarked, “Why, it’s a flying fortress!” — thereby instantly giving the plane its well-known nickname.