What is the Frame Rate of the Human Eye?

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

Is it, though? In this article we’ll try to figure out the ‘ideal’ frame rate for cinematography, using our own eyes as the guideline.

Is the frame rate of the human eye 24 frames per second?

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

Here are different frame rates, simulated:

4 frames per second:

12 frames per second:

24 frames per second:

48 frames per second:

You can see the flickering at all frame rates, can’t you?

One myth is that the human brain can process about 10-12 fps, which works okay 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.

Most certainly, we can put this myth to rest. The frame rate of the human eye is not 24 fps or 30 fps, and is definitely above 48 fps.

How much above 48 fps?

How does the eye see ‘frames’?

It’s a trick question. The eye doesn’t see frames like a camera does. We detect motion in very complex ways.

Open your eyes. Light enters and hits the retina continuously. There’s nothing to stop it from happening (except your eyelids). Imagine standing in the middle of a raging river. That’s exactly how the eye receives light when your eyelids are opened.

Whether or not the brain likes it, this information (light) is processed in near 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.

However, to throw a spanner in the works, 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)

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. 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. Note that there is no direct relation between cycles and frame rates in this context, this is just for fun!

In the beta state, it is 26 to 60 fps.

Let’s look at it from another angle.

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 in cinema. Here the duration of the gap is about 150ms, or 7 fps (rounded out). This won’t help.

Let’s try another angle.

Persistence of vision and the stroboscopic effect

When I was young, I was fed the lie that persistence of vision was the reason why we could watch cinema at 24 fps.

Persistence of vision is the optical illusion that occurs when the visual perception of an object does not cease for some time after the rays of light proceeding from it have ceased to enter the eye.

Although psychologists and physiologists have rejected the relevance of the theory of retinal persistence film viewership, film academics and theorists generally have not.

Film historians have often confused flicker fusion with afterimages that arise after staring at an object, while mostly ignoring the importance of the stroboscopic effect in their explanations of motion perception in film. The illusion of motion as a result of fast intermittent presentations of sequential images is a stroboscopic effect.

The stroboscopic effect is a visual phenomenon caused by aliasing that occurs when continuous rotational or other cyclic motion is represented by a series of short or instantaneous samples (as opposed to a continuous view) at a sampling rate close to the period of the motion.

Flicker Fusion Threshold

Flicker fusion threshold is the frequency at which an intermittent light stimulus appears to be completely steady to the human eye. The 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 15-60 fps.

Can we conclude that the frame rate of the human eye is 60 fps? Not yet.

Real world tests of the frame rate of the human eye

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 heightens the sense of reality for stereoscopic film. He tried that in Avatar. The highest frame rate I’ve seen in cinema is The Hobbit, at 48 fps.
Television sports used interlacing at 50i and 59.94i for years to keep the ball (whatever sport it might be) from blurring out. This motion contributes heavily to what many call the “video look”.
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 were 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.
The flicker fusion threshold does not prevent indirect detection of a high frame rate, such as the phantom array effect or wagon-wheel effect, as human-visible side effects of a finite frame rate were still seen on an experimental 480 Hz display.
It is well known amongst the gaming community that there is a big difference in 120 fps over 60 fps, not just in the gameplay experience, but visually as well.
In VR (virtual reality), a common baseline is 90 fps. However, one study titled Effect of Frame Rate on User Experience, Performance, and Simulator Sickness in Virtual Reality by Jialin Wang, Rongkai Shi, et al. concluded: “Our results show that 120fps is an important threshold for VR. After 120fps, users tend to feel lower SS symptoms without a significant negative effect on their experience. Higher frame rates (e.g., 120 and 180fps) can ensure better user performance than lower rates. Interestingly, we also found that at 60fps and when users are faced with fast-moving objects, they tend to adopt a strategy to compensate for the lack of visual details by predicting or filling the gaps to try to meet the performance needs.”
In some cases, it is possible to see flicker at rates beyond 2000 Hz in the case of high-speed eye movements (saccades) or object motion, via the “phantom array” effect.

Considering all of the above, it is hard to arrive at one standard frame rate for the human eye.