Images on your retina are reversed. Your retina “sees” everything backwards. Your brain reorients you. This image reversal is an adaptive advantage providing us with tremendous peripheral vision and the ability to view objects much larger than just a few millimeters.
The simplest illustration of how your retina sees is shown above. If you look at the eye chart, it gets turned upside down and reversed on the retina.
The image the retina “sees” is completely reversed.
The brain has to then reorient the image to allow you to see things “right side up” (and re-reversed).
Imagine you were looking through a keyhole trying to spy on someone inside a room. As you are scanning the room, looking at the right side of the room to the left side of the room, you really need to move your head/body in the opposite direction. This is the only way you can “see” the entire room.
The important point is that you can see the entire room through a very tiny hole. In the eye, the analogous part is the pupil.
Images need to be reversed so we can see objects much larger than the size of our pupil and so that we may have peripheral vision.
Using an example of the Washington Monument may help a bit. The image of the top of the monument must travel through your pupil and is focused on the inferior, or bottom, portion of your retina. The image of the base of the monument is focused (along with all the flags) on the superior, or top, portion of your retina.
What Does This Mean?
By reversing the image, we are able to visualize objects much larger than our eye. If you look at the light rays, colored in blue (see above), you will notice that the distance between the light rays emanating from the top and the bottom of the chart get closer together as they approach the eye. At some point, they actually cross and reverse.
The image of the eye chart is getting smaller, too. The light rays get smaller allowing the entire image to fit through the pupil and form a complete image on the retina, albeit upside down and reversed.
Without this reversal, we would have a very limited view of our world. It would be similar to viewing the world through a drinking straw.
(Note: For those of you who have had retinal detachment surgery involving gas injection, or macular hole surgery with gas or an intravitreal injection with an air bubble, this explains why the “gas/air” appears toward the bottom of your vision when looking straight ahead. With your head erect, the gas rises to the top of your eye, giving you the impression that the gas is on the floor,)