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Q & A: The Camera and the Eye

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Most recent answer: 09/15/2016
Q:
Dear Sir, What is the similarities and the differences between human eye and the camera? THANKS
- marram (age 10)
Kuwait
A:
Marram -

Whoa! Big question! I don't think I can get into ALL of the similarities and differences, but here's a few to get you started...

Both the human eye and a camera use something called a lens. In fact, they both use the same type of lens - a converging lens. Converging lenses are like the ones in magnifying glasses - they work to make an image look bigger. This is why you can see the details in something even if it's on the other side of the room. One thing about lenses is that they can only focus on things that are the same distance away from them. This is why your eyes can focus on things that are close to you or far from you; just not at the same time. Cameras are the same way - you can only focus them on things that are the same distance away.

In the camera, the lens focuses the light onto a piece of film. The film has chemicals in it that basically trap the image on it, making it permanent. Instead of film, your eye uses something called a 'retina.' The retina has lots of little tiny things called 'rods' and 'cones' all over it. These are basically tiny antennae that tell your brain about the light that hits them. The rods tell your brain if there's light in a certain spot or not (a bit like a black and white photo) and the cones tell your brain what color the light is.

There is one spot on the retina, though, that has no antennae at all. This is the spot where the nerve leaves your eye to go to your brain. At this spot, you can't see anything at all - it's called your 'blind spot.' This is one of the reasons that you have two eyes; what you can't see with one eye you can see with the other. Unlike your eyes, cameras have no blind spot, so they only need to have one lens.

Another important thing about seeing light is that you have to be able to control how much light gets in... otherwise, you couldn't see things in a brightly-lit room because you'd be overwhelmed by how much light there is. Your eye controls how much light gets in by changing the size of the pupil - the dark spot in the center of your eye. The more light there is, the smaller your pupil becomes, and the less light gets in. Many (but not all) cameras also can adjust to let different amounts of light in. This way, your outdoor pictures don't look washed out and your indoor pictures don't look too dark.

An interesting thing that I learned about pupils is that your pupil will also change size depending on what sort of mood you're in. But you sure can't tell the mood of a camera just by looking at it! Is it happy or is it sad? Hard to say...

There's lots of other similarities and differences between the eye and the camera, but I'm not going to try to explain them all. For more information, you can check out on the web. They have LOTS of good information on all sorts of things about both the eye and the camera. Good luck!

-Tamara

(published on 10/22/2007)

Follow-Up #1: eye's pixels

Q:
The best cameras in the market are around 10 megapixels. I have always wanted to know how many megapixels our eyes would be! I don't know if this is a stupid question, but it's one that really gets me thinking.
- Can (age 15)
Dubai
A:

Can- this is not a stupid question by any means! I can’t give a very precise answer, although probably if you were to ask a neuroscientist you could get one. What I can give is a technique to get an approximate answer.


Your eyes can’t see details that are too fine. So there’s something like an effective pixel size for the eyes: the smallest size they can resolve. How big is it?

The screen I’m looking at right now has around 1Megapixels. If I roll back to about a meter away, I can barely resolve the smallest features pictured on the screen: my visual pixel is then around the same size as the screen pixel. So at a distance of a meter, a screen with an area of around 0.1 m^2 has about 1M visual pixels. It then occupies about 10% of the main part of my field of view. (There is also a big peripheral field of view, but it’s not good for seeing details.) So I guess that there are about 10 M to 100 M pixels in the total field of view, densely spaced in the middle and sparser in the periphery.

I hope that this argument is useful to you, and that you can follow up with someone who actually knows something to get the numbers right.

Mike W.

p.s. I’m told that Vision Science by Palmer has a more detailed, accurate description of our effective visual pixel density. Apparently one of the main limits is not the eye itself but rather the density of brain cells devoted to various parts of the visual field.


(published on 10/22/2007)

Follow-Up #2: How many megapixels our eyes would be!

Q:
how many megapixels our eyes would be!
- dido (age 28)
India
A:
Lots!  The retina contains about 100 million rods and cones.  If you consider each of them as a pixel then the human eye has more than a Cannon.
It's a bit more complicated than that, since the rods and cones are not evenly distributed on the retina and each has a separate color response. 
See: 
for some more information.

LeeH

(published on 01/24/2010)

Follow-Up #3: Why do dim objects appear brighter out of the corner of your eye?

Q:
Why when we look at a direction our view of the opposite direction increases?!You can close left eye, look at the center then in a dark room put a Small Light Source where you can recognize it from left margin of the right eye's view then try to focus on it (you'll find nothing)Thank you so much
- Mahdi Islamzade (age 15)
Fars
A:

You're right, dim objects often appear brighter when you don't look directly at them. This is a well-known effect, and it's even useful to amateur astronomers.

Your eyes detect light with special light-sensitive cells called photoreceptors. These cells cover your retina (the back of the eye). There are two main types of photoreceptors: rod cells and cone cells, named for their shapes. There are also three different kinds of cone cells which are more sensitive to different colors of light. Your color vision comes from comparing the signals from the different kinds of cone cells. The cone cells are concentrated right in the center of your eye.

The rod cells can't see in color, but they are extremely sensitive. You use them for night vision. Unlike the cone cells, the rod cells are concentrated away from the center of your eye, in your peripheral vision. When you look to the side of a dim object, you move its image onto an area of the retina with more rod cells, which makes it easier to see. This is a useful technique for viewing very dim astronomical objects with a telescope. Looking slightly to the side can reveal a nebula or galaxy which is almost invisible when looked at directly.

Rebecca H.


(published on 09/15/2016)

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