Hey guys – why do we see a motion blur when moving hands?
I noticed the other day that when I move my hand quickly side to side but I’m not focussed on it- say when I’m talking to someone but using my hands to talk and gesticulating, that my hands trail slightly. There is a split second motion blur / smear behind my hands. Yet when I focus on my hands moving quickly, it isn’t as noticeable. Only when it’s in my periphery.
It is never a fully duplicate image / distinguished shape or prolonged, it’s just a smear.
What is the scientific explanation behind this? and why does it become more noticeable when you are aware of it?
Comments
[removed]
Your eyes are a kind-of-okay camera attached to an incredible computer that is your brain. The brain does a lot of work to make what your eyes are seeing look good, like making you forget that you’re blinking, smoothing out the shakes and jostles of your body while you move, etc. Have you ever noticed that if you tilt your head side to side, things still seem upright instead of rotating? (Unless you’re purposely looking for it, of course)
Motion blur is another consequence of processing vision. In the same way an incandescent lightbulb fades quickly rather than instantly turning dark when you flip the switch, a fast moving object will stimulate a bunch of the cells in your eyes very quickly, but then just as quickly be replaced by whatever is behind the object that’s moving. You end up with a smudge of where that object was, that we call motion blur. It’s nearly identical to the way cameras end up with motion blur.
It’s called persistence of vision.
It’s related to the fact that our retina is a bunch cones and rods all taking in information consistently and at different times/rates in a very very analog fashion. The information is for lack of a better word, burned into a cone or rod until everything that occurred is undone chemically.
Meanwhile, our brain is also taking in that information imperfectly as well. It is trying to translate it all as best as possible while constantly getting bombarded with new information.
As a result we end up sorta having an interpretation that is kinda after image-y or motion blurry.
We have many optical illusions that we utilized to make cool or fun effects as a result. That being said, if it is exceptionally smeary, it might actually be some pathology or issue.
Just think of your eyes as a camera and your brain as watching a movie they’re capturing. Cameras will capture a video with certain frames per second and certain shutter speed. During the shutter they collect light and imprint that light into the image. If the object move during the shutter, it will aprear smeared over its trajectory from the beginning of the shutter and the end.
Eys are similar, they just don’t have an exact shutter, but more of a fadeout (called the persistence of vision). When a light hits a receptor in your eye it will make a color dot in your vision that then slowly fades. This will have the same motion blue and long shutter in a camera, just more smoothly fading into the past.
If you are in artificial flickering lighting, that only shines short pulses of light, but still multiple of them within the fade time of your receptors, you could wave your arm around and see multiple copies of it, as the flickering light just instantly imprints a snapshot of your hand at different places, before the previous snapshots fade out.
This fading “shutter” of your eyes is also why you are even ablew to see CRT images at all. If you look at them in slowmo, you can see a bright dots just scanning the lines from left to right and then from top to bottom, but the stimulation of your eyes can reamin until it gets back the in the next frame, so you can see the whole image. Actaully the CRT screen itself has a similar fade like your eyes, jsut much quicker. As in the slowmo you could see that it’s not just a single dot with blackness behind it, but that at least the current line remains there and only fades out when the next line is being drawn. So you can just think of your eyes react to that CRT dot the same exact way as the screen itself, but slower enough that it would only begin to fade when the dot draws all the lines and gets into the next frame. And of course in real life they are getting updated everywhere at once, while still retaining some of the previous stimulation that fades out.
[removed]
At any given point, your brain is perceiving a short interval of time rather than a single instant. This is because there’s not really a distinction between live experience and memory over a duration below say 0.1s (and of course live experience is itself limited by the speed of nerve impulses).
You see a blur because, for a given point in your field of view, you see your hand for part of the interval, and the background for the rest of the interval. This make for a perception of averaging transparency, like a blur.
As for why the experience is different in your peripheral vision, that may be down to you naturally focusing less on things in your peripheral, making for a longer perceptive interval.
Your hand moves so fast that your eyes can’t catch up
It is called persistence of vision. Your brain retains the image perceived by your eyes for a fraction of a second, which is how you see motion. If something is moving fast enough, your brain is going to overlap those images, and you see motion blur.
It’s the basis on which film and animation operates. The 24frame per second rate of film speed is roughly in line with our own persistence of vision. Which is also why the animation in some cheaper productions appears so poor. They often hold images on 3s or 4s, creating a frame rate of 6-8fps which is below our persistence of vision.
Other people have explained this already, but here’s a cool experiment you can do to illustrate it:
Look up at a ceiling fan that is set to max speed. It just looks like a circle blur, right? Now keep looking at it but blink rapidly. Those brief moments that your eyes are closed are enough for your brain to “clear” the data from the previous instant, so it will look almost like stop motion instead of a smooth blur, and you’ll be able to see each individual fan blade.