Why You Seem to Move Slower When Shooting at Longer Focal Lengths

This old but no less informative video has recently resurfaced, and it illustrates an interesting optical illusion that makes speed appear much slower when using a longer focal length.

The video, spotted by resurfaced on Laughing Squid, features Akiyoshi Kitaoka, a professor of psychology at Ritsumeikan University in Kyoto, Japan. Professor Kitaoka has a website full of optical illusions. Additional motion-based optical illusions are available on Kitaoka’s website, although visitors should be warned that there are extensive flashing scenes.

Scientist, author, and educator Mark Changizi is an expert on optical illusions, and he recently took another look at Professor Kitaoka’s video. In the description of Changizi’s recent video above, he explains that he wrote to Kitaoka in 2020 when Professor Kitaoka shared the optical illusion on Twitter.

“The illusion that speed decreases when zoomed is because when one focuses on an inner portion of the movie, the optic flow angular speed is slow and appears to fill one’s entire visual field, which is consistent with overall lower forward speed,” Changizi explains.

“The more zoomed, the more densely packed the overhead rigging appears. So, even though you appear to be moving forward more slowly when zoomed in, the actual rate of rigging flowing by remains constant, consistent with the same forward speed in all conditions,” Changizi continues.

He adds that “these optic flow dynamics” explain many optical illusions that people experience. Changizi says that the brain frequently perceives anticipated movements to correct for neural delay.

In the TED-Ed presentation below, Changizi explains more about optical illusions and how the brain has evolved to deal with natural stimuli rather than the artificial and unnatural stimuli people experience daily.

Given that the human brain evolved over millions of years — lumping multiple human species together, as homo sapiens are a relatively young primate — the brain cannot evolve anywhere near quick enough to fully deal with the rapid change in how people move around the world and the things people encounter.

Human vision is remarkably sophisticated and complex, but certain relatively simple optical illusions confuse the brain.

In Kitaoka’s example, the relative density of reference points changes as the lens zooms in, shifting the apparent angular speed and making forward velocity appear lower.

This is a common situation for video gamers when adjusting their field of view, especially when playing first-person shooter or racing games. The example video below from John Daniels on YouTube illustrates how sense of speed changes with field of view.

Using a train simulator game, Daniels continually adjusts the field of view while the train maintains a constant speed. Starting with a much wider field of view and slowly shifting toward a narrower field of view, like going from a wide-angle lens to a telephoto lens, the sense of speed changes dramatically.