Notes by Dr. Optoglass: Motion and the Frame Rate of the Human Eye

Topics Covered:

  • Electroencephalography (EEG) and Beta waves
  • Phi and Beta phenomenon
  • The Frame Rate of the Human Eye

Photography is truth. The cinema is truth twenty-four times per second – Jean-Luc Godard

Warning: Don’t look if you are prone to seizers!

4fps12fps24fps48fps

Imagine standing in the middle of a raging river, with water flowing towards you. That’s exactly how the eye works when your eyelids are opened. Light enters and hits the retina continuously – there’s nothing to stop it from happening.

Whether or not the brain likes it, this information (the unstoppable raging water) is processed (sampled) in real time. In other words, it’s not the eye that does this, but the brain. One of the techniques scientists use to study how neurons in the brain work is called Electroencephalography (EEG).

Electroencephalography (EEG)

The patterns that emerge from EEGs clearly show how the brain has to sample data that the eye throws at it, and by looking at the sampling frequencies involved, one can find out what the ‘refresh rate’ of the eye is. It appears that the waves from an EEG are not a single type, but many. These varying frequencies are grouped together in ‘bands’ for easy identification.

The main bands are as follows:

  • Alpha (8-13 Hz)
  • Beta (13-30 Hz)
  • Gamma (30-100+ Hz)
  • Delta (upto 4 Hz)
  • Theta (4-8 Hz)
  • Mu (8-13 Hz)

Brain waves

The three most particularly interesting bands are the alpha, beta and gamma band. Alpha is the band that traditionally represents the rest state (Theta is the state that corresponds to the meditative state). Beta is the active zone – the one that humans use in day to day activities. Gamma is a mystery – it is thought to be the wave responsible for delivering the continuous perception we take for granted, but there is no factual evidence that definitively supports this.

In any case, we can see that in a normal waking human, the brain ‘refreshes’ from 4 Hz to 100 Hz. Professor Sampler must have told you that a Hz (or a cycle) is a crest and a trough in a wave. Since frames as we know it in cameras have only a positive state, we can confidently claim that the frame rate of the brain lies between 8 to 200 fps, depending on the state it is in. In the beta state, it is 26 to 60 fps.

A frequent belief is that the human brain can process about 10-12 fps – which works well when each frame is just an iteration of the previous one. But what if each frame is totally dissimilar, as in the flickering images at the beginning? It must be obvious that in such a case, 10-12 fps won’t cut it. In fact, even at 48fps, the colors don’t completely blend and fuse into one color. The example had to be limited to 48/60fps, since most computer displays don’t refresh faster than 60fps.

Phi Phenomenon and Beta Movement

To keep it simple, phi phenomenon is an optical illusion that causes a perception of constant movement from a group of frozen images at a certain speed of images per second. Beta movement, on the other hand, is an optical illusion that give the appearance of motion due to rapidly changing ‘static’ images. The difference between phi and beta (this beta has nothing to do with beta waves discussed above) is also a subject of misunderstanding, even among experts. The difference is psychological. In phi phenomenon luminous impulses change in sequence, i.e., lights go on and off at regular intervals; whereas in beta movement lights do not move, but seem to.

Suffice to say, a combination of phi and beta phenomenon is what causes our perception of motion cinema.

Flicker Fusion Threshold

Flicker fusion threshold is the frequency at which an intermittent light stimulus appears to be completely steady to the human eye. My example at the beginning is a simplified version of this. Unfortunately, there are six parameters that can change this value:

  • Frequency of modulation
  • Amplitude of modulation – Intensity of illumination
  • Maximum illumination intensity
  • Wavelength of illumination
  • Position of the retina at which stimulation occurs
  • Degree of light or dark adaptation

The maximum fusion frequency for rod-mediated vision is about 15 Hz. The maximum fusion frequency for cone-based vision is about 60Hz. This corresponds to between 30 to 120 fps.

Practical Testing

Various agencies have conducted tests of their own to find the ideal frame rate for cinema.

  • Thomas Edison believed that 46 fps was the minimum for strain-free vision.
  • James Cameron believes shooting in 60fps will heighten the sense of reality for stereoscopic film.
  • Douglas Trumbull, the developer of Showscan, discovered that as the speed of projection ramped up, so did the emotional response, peaking at 72 fps. In fact, movies shot on 24 fps are usually projected at 48 fps or 72 fps.
  • BBC Research had successfully demonstrated in 2008 that increasing the frame rate can significantly improve the portrayal of motion even at standard definition. Their tests were conducted by shooting 300 fps on a Phantom V5.1 camera, but displayed at 100 fps on a Christie Mirage S+4K projector due to limitations in display technology.

Considering all of the above, I believe the frame rate of the human eye is about 120 fps.

Takeaways:

  • Through EEG, we know that beta waves lie between 13 to 30 Hz, which corresponds to 25 to 60 Fps.
  • Through phi phenomenon and beta movement, the maximum fusion frequency of the eye corresponds to 60 Hz, or about 120 fps.
  • Practical testing by eminent agencies have shown that the frame rate must be above 60 fps, and even above 100 fps to take full advantage of the high spatial resolutions.

Links for further study:

Next: Stereoscopy – Understanding Depth
Previous: Color Bit Depth, Middle Grey and White Balance of the Human Eye

2 replies on “Notes by Dr. Optoglass: Motion and the Frame Rate of the Human Eye”

  1. I was playing a game on my phone, and it seemed that the frame rate appeared to be higher if I held it about 15 inches away, compared to holding it around 9 inches. Is there a reason for this? Does it have to do with the different rays? I can’t find anything on this so any answer would be greatly appreciated.

  2. I have a question about the flicker-fusion threshold. When I read about that, some said the limit of human’s eye is 25 Hz, other said 60 Hz… What is actual human’s the flicker-fusion threshold 25 Hz or 60 Hz? I guess there are different test mode so the number might be different. Please, answer this problem.

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