In the 21st century moving pictures are everywhere, from the big screens in cinemas displaying the latest blockbuster to animated gifs and ad banners on the web. But how is it possible for a series of still images to become “alive”? It may seem magic, but the heart of how moving pictures are achieved is based on something everyone who can see has encountered numerous times – fast moving objects appear blurry. The reason their blurry is because the human brain can only process so much from our eyes at any one time. If the brain could process more than it does now per moment fast-moving objects will appear less blurry. And if that is possible we can make that object out or clearly and react accordingly to it quicker. Many animals have faster vision processing, which has allowed them to react quickly to fast-moving dangers (such as a predator or a vehicle). In the 19th century, some people “discovered” this and worked out that the blurring is the result of the brain processing scenes at regular intervals. Then some of them worked out that if you showed a still image to someone for a split second and then took it away that image could still linger in that person’s mental visual picture for a moment after the image was taken away. Then they worked out that if (after taking away that first picture) you then showed a second picture that is slightly different they blur together. If you can repeat this for, say about 24 times a second, you can create the illusion of moving images in the viewer’s mental picture. Many devices and means exploiting this discovery were developed in the 19th century, but the one that easily demonstrates how this works (and made moving pictures popular) is the celluloid movie film strip (as seen here).
Frame
Rate
The still images that make up a moving picture are called “frames.” The speed a moving picture records or displays these frames is measured by frame rate. The minimum frame rate you can achieve the illusion of movement on screen (for human eyes) is around 10 to 12 frames per second. In traditional film movies, the standard frame rate is 24fps. This came about in the mid-1920s with the advent of sound in movies. Before this, 16fps was the average. I say average because cameras and projectors then were hand-cranked machines, which meant that the frame rate was not steady and universal.
“THERE IS NO SUCH THING AS A SET CAMERA SPEED!” – From a 1915 projectionist's handbook. Many projectionists at this time tended to crank their projectors quickly to squeeze in as many showings as possible in a day.
According to Thomas Edison (who did some early work on movie film technology) the ideal rate for film to create the illusion of movement on screen (and remove flicker) is 46fps. "Anything less will strain the eye."
Filmmakers have tinkered with the frame rate of their cameras to alter the speed of the actions been recorded. A movie recorded at a faster frame rate and projected at normal speed will create “slow motion” on screen. A lower frame rate been projected at normal speed will result in the action speeded up.
In 2011 researchers at MIT had made a camera that recorded at 1trillion fps. With this camera they captured the movement of a beam of light entering an empty bottle. Faster cameras have been built since then.
Some film directors have recently experimented with higher frame rates to reduce motion blur and flicker. Peter Jackson filmed his Hobbit trilogy (2012-14) at double the standard film rate (48fps). When viewed at this rate, viewers experienced pictures that looked more like high-definition video than traditional film, with one projectionist saying “It looked like a made-for-TV movie.”
“Film is truth 24 times a second.” – Jean-Luc Godard, French-Swiss film director
https://en.wikipedia.org/wiki/Frame_rate
Image
Registration
Images can still linger in your mind’s eye for moments after seeing it. You can notice this when staring at a block of solid colour or a light source for a while then looking away. This is due to the rods and cones in the retinas of your eyes “relaxing” during your stare and the lag in their reaction from moving your gaze to them registering the change of scene. Within the transition from one frame to the next, your retinas is are in the mist of processing the change in scene. This results in your mind processing this lag as the blur of action that happened in-between the frames.
https://en.wikipedia.org/wiki/Afterimage
Flicker
As moving
pictures involve showing a succession of still images, one obvious problem is the
issue of flicker. Anyone who has played with a strobe light will know about
this. If you slowly increase the rate of flashes in a strobe light they will
come a point when the human eye stops seeing intervals of no light and sees
only a continuous bright light. If a movie shows its frames below a certain
frequency the viewer will see the action on screen in a jerky manner, ruining
the illusion.
In the
motion picture industry, the human threshold is usually seen as 16Hz. It is
because below this point the blacking out of the picture caused by the rotating
shutter becomes noticeable (and annoying). To prevent flicker, most moving
pictures are usually made using frame rates higher than 16 frames per second.
https://en.wikipedia.org/wiki/Flicker_fusion_threshold
Animal
Vision
Some animals can process the sight of fast moving objects faster than humans. All birds (particularly birds of prey) have a higher “flicker threshold” than humans. Pigeons alone have a flicker threshold of 100Hz (compared to 60Hz in humans). Many mammals have eyes that could too (so far we know conclusively dogs can). Such highly reactive eyes have allowed such animals to pull off incredible feats of obstacle avoidance and manoeuvrability at speed.
Have you noticed that birds are able to move out of the way of vehicles on the road at the last moment? They can do so because of their fast visual processing. However, if you show birds a movie made for human eyes, they’ll not see the illusion of movement. They’ll see a dull slideshow.
Flies also have faster eyes that humans.
Pokémon fans may recognize that this flying-type is doing the “aerial ace” attack. This attack involves flying very quickly and fast responses to obstacles. This can only be possible because birds have faster image processing than humans.
Intervals
(Shutter)
If a projector just projected what was been fed continuously the result will be a very blurry mess on the screen. To make sure what appears on screen is just the frames of footage, a rotating shutter is used in the projector to break the projected beam continuously at moments when the film is been moved along a frame.
Repeating
Frames
To also help reduce flicker the shutter can be made to show each frame twice or three times, increasing the frame rate on screen. In animation it is common practice to “shoot on twos,” meaning they photograph each frame of footage twice to create movies at the full standard 24fps with half the effort.
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