The speed of human visual perception is a surprisingly tricky subject, with no single answer. It depends on what type of speed you want to look at. Here we will break down 5 fascinating facets of vision and give answers to the current known speed limits of what we can see – enjoy!

1. What Is the Shortest Amount of Time in Which We Can Perceive a Flash of Light?

Like many interesting answers in science, this came incidentally and turned out to be way beyond what anyone would have imagined.

Multiple astronauts in outer space reported seeing unexplained anomalies of extremely brief spots and fleeting streaks of light, even with their eyes closed. These would occur every few minutes. It turned out they were seeing a recently discovered phenomena called high energy cosmic particles. These are by far the fastest moving mass in the known universe.

The exotic particles come from somewhere in our Milky Way galaxy, but can travel so close to the speed of light, that the energy required to accelerate them is beyond anything known in astrophysics – not even two super massive black holes colliding would get close! When they hit the earth’s atmosphere, their energy is transformed into myriads of new particles (E = mc²).

The point here is that at this extreme speed, the time needed to perceive such high energy particles via human vision is on the order of femtoseconds - literally quadrillionths of a second!

The brighter the flash the less time is needed to detect it. In outer space there is negligible light pollution, so contrast between light and dark is close to its possible maximum.

However currently there is no understanding of the neurobiological mechanisms of seeing high energy particles. It likely involves the fact that millions of different neurons can potentially pick up on a sensory input, but the less that do, the less accurate the perception is. Some research shows that our eyes can potentially detect a single photon, but this is a probabilistic event.

2. How Many ‘FPS’ Do We Need to Perceive Fluid Motion?

‘Frames per second’, commonly known as ‘FPS’, is the common benchmark for our visual technologies like computer screens, TVs and cameras. It turns out to be a useful measure for humans too.

The answer to this question was discovered at the end of the 19th century following the invention of cinema. Early cinema companies found that 16 FPS was the bare minimum that could trick the brain into seeing movement on-screen as continuous. Here we can get an idea of how of our brain glues together separate images into motion at the speeds of movies (24 FPS) and typical computer displays (60FPS).

This is surprisingly low, for example many animals, such as birds, require at least 75 FPS just to perceive motion. For them, looking at a TV is the same as seeing a sequence of separate static images.

This human ability actually represents something deep about how little visual information our brain needs to extrapolate sensory information. Here is an example from psychophysics study.

As you can see, we can build an internal model of motion from a surprisingly small amount of cues – our visual centers do the heavy lifting for us!

3. How Many Flashes of Light per Second Are Needed See Light as Continuous.

The answer here is called the Flicker Fusion Threshold, and also comes from early pioneers of cinema. Even though motion could be perceived at 16 FPS, the flashing of the screen at this rate was considered unbearable. Here is an example from the first film ever made.

To solve this problem new shutter technology was developed to actually flash each individual image frame multiple times in succession. The rate at which the flashing became comfortable and relatively unnoticeable, was around 46 flashes per second.

Therefore, our ability to consciously perceive pulses of light operates at roughly three times speed of our ability to motion in terms of FPS. In short, we are much more sensitive to luminance than motion.

4. How Quickly Can We Perceive an Absence of Light?

In others words, how fast can we detect a flash of black among continuous light? Research with younger and older people was done in this area to investigate the degeneration of vision with aging. Detection was found to be on the timescale of 18 milliseconds for younger adults, and 22 milliseconds for older adults.

This translates to between 45 and 55 FPS, when 1 dark frame becomes unnoticeable, or flicker free. The caveat here is that this was the average, but individual results varied significantly.

Even in a small study group, some participants were able to spot a single black flash among 500 FPS of light flashes (or 2 milliseconds). Again, this is extremely fast. In comparison it takes 100-400 milliseconds to blink, which is commonly discounted from conscious experience.

5. How Long Do We Need to See a Scene in Order to Be Able to Identify It?

Research from 2014 into rapid image recognition discovered that we can detect specific scenes on much shorter timescales than previously thought. In experiments where different randomized images (e.g. a house, a dog, a forest etc.) are flashed in rapid succession, subjects could recognize or recall a specific scene in as little time as 13 milliseconds.

This translates to being shown 75 different images in a single second, and being able to say with some reliability if a specific image was there, or not. You can see for yourself just how remarkably speedy this really is.

Interestingly this wasn’t found to be the limit, it was simply the fastest the researchers could display images at the time.

Well there we have it, hopefully you learned that human vision is deeper and more mysterious than you once thought, and that it is genuinely remarkable. If you’d like to delve deeper, then this great YouTube video by TechLaboratories goes into more depth.