Placeholder Image

Subtitles section Play video

  • Watch the center of this disk.

  • You are getting sleepy.

  • No, just kidding.

  • I'm not going to hypnotize you.

  • But are you starting to see colors in the rings?

  • If so, your eyes are playing tricks on you.

  • The disk was only ever black and white.

  • You see, your eyes don't always capture the world as a video camera would.

  • In fact, there are quite a few differences,

  • owing to the anatomy of your eye

  • and the processing that takes place in your brain

  • and its outgrowth, the retina.

  • Let's start with some similarities.

  • Both have lenses to focus light and sensors to capture it,

  • but even those things behave differently.

  • The lens in a camera moves to stay focused on an object hurtling towards it,

  • while the one in your eye responds by changing shape.

  • Most camera lenses are also achromatic,

  • meaning they focus both red and blue light to the same point.

  • Your eye is different.

  • When red light from an object is in focus, the blue light is out of focus.

  • So why don't things look partially out of focus all the time?

  • To answer that question,

  • we first need to look at how your eye and the camera capture light:

  • photoreceptors.

  • The light-sensitive surface in a camera only has one kind of photoreceptor

  • that is evenly distributed throughout the focusing surface.

  • An array of red, green and blue filters on top of these photoreceptors

  • causes them to respond selectively to long, medium and short wavelength light.

  • Your eye's retinas, on the other hand, have several types of photoreceptors,

  • usually three for normal light conditions, and only one type for lowlight,

  • which is why we're color blind in the dark.

  • In normal light, unlike the camera, we have no need for a color filter

  • because our photoreceptors already respond selectively

  • to different wavelengths of light.

  • Also in contrast to a camera,

  • your photoreceptors are unevenly distributed,

  • with no receptors for dim light in the very center.

  • This is why faint stars seem to disappear when you look directly at them.

  • The center also has very few receptors that can detect blue light,

  • which is why you don't notice the blurred blue image from earlier.

  • However, you still perceive blue there

  • because your brain fills it in from context.

  • Also, the edges of our retinas have relatively few receptors

  • for any wavelength light.

  • So our visual acuity and ability to see color

  • falls off rapidly from the center of our vision.

  • There is also an area in our eyes called the blind spot

  • where there are no photoreceptors of any kind.

  • We don't notice a lack of vision there

  • because once again, our brain fills in the gaps.

  • In a very real sense, we see with our brains, not our eyes.

  • And because our brains, including the retinas,

  • are so involved in the process,

  • we are susceptible to visual illusions.

  • Here's another illusion caused by the eye itself.

  • Does the center of this image look like it's jittering around?

  • That's because your eye actually jiggles most of the time.

  • If it didn't, your vision would eventually shut down

  • because the nerves on the retina stop responding to a stationary image

  • of constant intensity.

  • And unlike a camera,

  • you briefly stop seeing whenever you make a larger movement with your eyes.

  • That's why you can't see your own eyes shift

  • as you look from one to the other in a mirror.

  • Video cameras can capture details our eyes miss,

  • magnify distant objects

  • and accurately record what they see.

  • But our eyes are remarkably efficient adaptations,

  • the result of hundreds of millions of years

  • of coevolution with our brains.

  • And so what if we don't always see the world exactly as it is.

  • There's a certain joy to be found watching stationary leaves

  • waving on an illusive breeze,

  • and maybe even an evolutionary advantage.

  • But that's a lesson for another day.

Watch the center of this disk.

Subtitles and vocabulary

Operation of videos Adjust the video here to display the subtitles

B1 US TED-Ed photoreceptors camera eye blue light capture

【TED-Ed】Eye vs. camera - Michael Mauser

  • 20683 1595
    稲葉白兎 posted on 2015/05/03
Video vocabulary