I was reading about the Camera Obscura and the physics of apertures, and I read that "The camera obscura produces a flipped (horizontally) image as well as an inverted (vertically) one."

I understand why condensing light through a pinhole would flip an image, and it makes sense why it would be inverted both horizontally and vertically, as a pinhole is symmetrical.

Part of the eye is the pupil, which can be accurately modelled as an aperture/pinhole. And I have always been taught in school that the brain inverts the retinal image vertically.

My question is this: why doesn't the brain also have to flip the retinal image horizontally as well? What concept am I not understanding correctly? Does the fact that the eye also includes a lens behind the aperture affect this problem? Or is what I have been taught in school a simplification? What about the fact that the brain has two eyes and two visual fields split across the two eyes?

  • $\begingroup$ I read that post and it makes sense, but the perceptual illusion only occurs due to you standing in front of the mirror, your retina should be essentially "behind the mirror" (behind the image plane) so to speak so I don't think it directly answers my question $\endgroup$ – Michael Sohnen Feb 2 '18 at 9:31
  • $\begingroup$ @Farcher Your links are irrelevant. This is mot what OP is asking about. $\endgroup$ – safesphere Feb 2 '18 at 9:32
  • $\begingroup$ If the pupil flips the image both vertically and horizontally, but you see it correctly, then the brain flips it back both vertically and horizontally. Don't believe everything you hear in school. $\endgroup$ – safesphere Feb 2 '18 at 9:34

The brain does flip the image horizontally, too, but talking of "flipping" in the first place gets a bit into pseudoscience, since the brain doesn't really do any "flipping" of images or have any a priori awareness of how the retina is constructed - it simply learns by experience (as a baby) that moving the left hand leftwards, produces a change in certain inputs, and that all things that lie on the left-hand side of the visual field correspond to these inputs - inputs that only adults know lie on the right-hand side of the retina. To the brain, those right-sided inputs in the retina are the left-hand inputs, without any correction needed.

That's also why the "blind spot" at the optic nerve is invisible, because the brain simply has no inputs from that area, and two-eyed people can always see the full field of view, so nothing is routinely seen to "disappear".

You can see your entire fingertip disappear if you look at it steadily off-centre, out of one eye, with the other closed, from about a foot away - and if you wiggle the fingertip, it cannot be seen despite being in front of you.

But we so rarely allow (or need) those circumstances with our eyes to coincide that it rarely has any bearing and is simply unnoticed.

Edit: It's quite curious actually. At a little over a foot away and looking through my left eye only, I could wiggle my left index finger significantly in a beckoning fashion, and I cannot see my finger at all (my palm appears to have 3 fingers and a thumb), although I can see the changing shadow that the movement casts on my thumb (because of the window to my right).

When my finger leaves the blind spot, it appears to fade in from the background, and the background (which is only vague anyway when viewed off-centre) is interpolated from my wider field of view.

For anyone interested in where to find the blindspot in the field of vision, the blind spot in my left eye roughly coincides with me stretching my right arm out in front of me, with my palm facing the floor, and the fingers of my right hand splayed, and if I keep the central focus my left eye on my right little fingernail, my right thumbnail is in the blindspot and cannot be seen.

  • $\begingroup$ Thanks for the response, i perhaps should not have posted such an unscientific question, but at least I learned something and confirmed that aperatures invert images horizontally as well (and presumably at all angles) $\endgroup$ – Michael Sohnen Feb 2 '18 at 9:51
  • $\begingroup$ @MichaelSohnen, a question about optics seems scientific enough for me! $\endgroup$ – Steve Feb 2 '18 at 9:59
  • $\begingroup$ Your first sentence contradicts itself. It would be better to simply start with, "the brain doesn't really do any 'flipping'..." $\endgroup$ – Solomon Slow Feb 2 '18 at 14:35
  • $\begingroup$ @jameslarge, I see your point, but I was initially trying to answer from a purely optical (and ocular) point of view and work with the OP's conception of the situation - and to confirm that, indeed, the image is flipped on both axes. I said more than enough afterwards about why it was wrong to conceive of the brain then un-flipping or reversing this image. $\endgroup$ – Steve Feb 2 '18 at 15:00

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