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As far as I'm aware, the eye acts like a pinhole camera in that it inverts the image on the rentina. This makes sense as the rays converge and form a focal point that is upside down.

Myopia (shortsightedness) is described as the rays focusing before the retina, resulting in a blurred distance image. This would still be the right side up. However with Hypermetropia (farsightedness) the focal would be behind the retina, so the rays should project a blurry but right side up (but interpreted by the brain as upside down) image.

I know this is not the case, by why?

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    $\begingroup$ In both the cases, blurring is due to non-focused rays on the retina, but the image is inverted (w.r.t the object, i.e. inverted once by the eye lens) in both the cases. Brain reads the inverted image to be the erect one. I guess you are confusing the inversion of image with non-focused rays which cross before or after the retina. However, these non-focused rays are rays from a single point and hence cannot show inversion. $\endgroup$
    – stochastic13
    Oct 11, 2013 at 11:17
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    $\begingroup$ Don't they invert after the focal point? This is what it shows in the ray diagrams. $\endgroup$
    – Richard
    Oct 16, 2013 at 10:24
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    $\begingroup$ There are actually two types of ray diagrams. One which shows two rays, one from the top and one from the bottom of the object, they cross and form an inverted image on the retina. This type cannot help you visualise myopia/hypermetropia. The other type is two rays from the same point diverge and meet exactly at the retina in normal case. They show the focus and not the inversion. It is these rays which converge before or after during defects, hence the image is focussed but its inversion is unaffected. Tell me if you still have any doubts. $\endgroup$
    – stochastic13
    Oct 16, 2013 at 13:26
  • $\begingroup$ This answer may help you understand the difference between blurring and inversion (image below the axis while object, a point there, above it). $\endgroup$
    – Andrestand
    Mar 7, 2014 at 10:56
  • $\begingroup$ I know this post is old, but I had exactly the same question.I'd like some clarifications.I've done a sketch of a vertical object placed very close to the eye (closer than its focal length). 2 rays from the top of the object reach 2 different spots on the retina so that the object's image on the retina covers almost all the retina. A ray from the bottom of the object hits the retina in its middle, hence the blurriness of the image. But it seems to me that the upper part of the retina gets an upward image while the bottom part gets an inverted image; this can't be right. What's wrong with this? $\endgroup$
    – untreated_paramediensis_karnik
    Dec 13, 2015 at 15:50

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The eye does not act as a pinhole camera. It is a multi element optical system with the cornea and inter-ocular lens doing most of the work. An image is inverted because the light entering the eye from above is headed DOWN, below the optical axis and will therefore image on the lower half of the retina. The light coming from below is has a positive slope and will image on the top half of the retina. This has nothing to do with focus. Regardless of where your image plane is (retina), the converging cones of light are still headed up or down depending on where they came from in object space. While walking on the sidewalk, the light from the gum you're about to step in is headed UP towards your eye and will image on the upper half of the retina. The light from the bird nest that is about to hit you is coming down to you from above and will continue that trajectory even as it passes through the lens in your eye. This has nothing to do with focus.

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