Recently, I learned that the image of the world which we see actually reaches the retina inverted. This is due to the fact that the retina lies behind the focus of our eyes.

I have two questions:

  1. When the retina lies behind the focus and the lens in our eyes has a spherical shape, shouldn't left and right also be interchanged?

  2. What is the advantage of this? Why does the retina lie behind the focus and not in front of it? Or doesn’t it matter since our brain is not a real screen and it’s only a question of the correct interpretation of the electic signals?

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    $\begingroup$ The reason lies in the physics of lenses. There is no advantage for us. $\endgroup$
    – Chris
    Aug 23, 2017 at 12:38
  • $\begingroup$ As is said, i totally understand the reason (i.e. the physics going on). But i only read that up and down interchange and not that left and right do the same which makes for me no sense. $\endgroup$
    – Max W
    Aug 23, 2017 at 12:42
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    $\begingroup$ I am not sure what you mean with:"the retina lies behind the focus of our eyes", but normal eyes (or those corrected with glasses) have the focus point on the retina. Convex lenses project images inverted (up-side-down, and left-becomes-right). This is how lenses work, a law of nature. We as organisms can either use it and correct it in our brain or develop something better (which is in evolution not always possible). So there is not an advantage above normal projection, because normal projection is never developed in the course of evolution. $\endgroup$
    – user35628
    Aug 23, 2017 at 14:11
  • $\begingroup$ If the focus is exactly on the retina, then how should the brain know where the light originally came from. I mean if the light is not separated in space, all information is gone and you left with a bright point. $\endgroup$
    – Max W
    Aug 23, 2017 at 14:30
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    $\begingroup$ @MaxW That isn't what in focus means. Focus is about taking all the light originating from the same point and putting it in the same place on the surface of the retina. Not putting all of the light from everywhere in the same place. The only way to do this is with inversion. "according to your opinion" doesn't change physics. $\endgroup$
    – Bryan Krause
    Aug 23, 2017 at 23:12

1 Answer 1


There may be no advantage of the inverted image on the retina. But there is an advantage with the lens that refractes the image inverted.

Our eye lens is of convex in nature. Now lets see what is the advantage of convex lens being our eye lens.

enter image description here

Source : http://www.ivyroses.com/HumanBody/Eye/concave-and-convex-lenses.php

The light from the object, when reaches the convex lens it gets refracted and gets "converged" and is a "real image" and also "diminished" but the problem is it makes the light inverted.

Now let us see what happens if our eye lens is a concave lens.

enter image description here

Source : http://www.ivyroses.com/HumanBody/Eye/concave-and-convex-lenses.php

In this case, when the light from the object from a distance reaches the lens, the lens "diverges" the light, also it "enlarges" the image, but it produces an "erect" image but the major problem is that the image is "virtual".

We know how evolution works, it selects more advantageous option generally.

Case - 1: If the lens is convex, the only problem the eye has to handle is that the image is inverted. Case - 2: But if the lens is concave, it should converge the diverged light, it should diminish it and it should also make it real.

If we take both cases into consideration, case - 1 seems to be easily handled, that's why we have inverted image falling in the retina. There maybe no other specific advantage associated with it.

  • $\begingroup$ I totally understand that the lens should be convex, this is not my question. You could simply place the retina a bit in front of the focal point and the picture would not be flipped. So, does this little extra space of moving the retina away from the lens give any optical advantage? $\endgroup$
    – Max W
    Aug 23, 2017 at 14:53
  • $\begingroup$ Say me the exact point where the retina should be placed so that convex lens gives an erect image, at f or between between f and 2f or where exactly? I have been taught that you get an inverted image where ever you place the screen. Refer - physics.tutorvista.com/light/convex-lens.html . Only when the object is at 0cm to 25cm, you will get an erect image but that will damage the lens. $\endgroup$
    – Kawin M
    Aug 23, 2017 at 15:56
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    $\begingroup$ @MaxW That's not at all how it works. Whether in front or behind the focal plane, the image will not be in focus; it will be blurry. The pictures in the answer here are showing light from one origin, one object, not light from different objects. It is flipped as soon as it goes through the lens. $\endgroup$
    – Bryan Krause
    Aug 23, 2017 at 23:15

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