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Vision & Eyesight: Limitations & Oddities

Other than being blind in at least one spot on your eye (due to where the optic nerve attaches) it turns out there are several reasons we can not trust our eyes.

Persistence of Vision
The human eye does not instantly forget what it saw.  It takes at least 1/24th of a second to forget what it saw.  Television and movies only show us a new picture every 24th of a second.  Since our eyes didn't forget the previous image yet, the new image gets blended with the previous.  This gives us the illusion of smooth, realistic motion.  If an image is really bright, it may leave a trace on your eye for a much longer period of time.  Seeing spots after someone takes a flash picture is a good example of this.
3D Effects
3 Dimensional Cross eyed viewing
We see things in 3 dimensions (3D) because we can see them from two points of view at the same time.  Our left eye gives us one point of view, while our right eye provides the other.  Our brain overlaps these two points of view into one image that contains depth.   The images above look the same but are actually just a little different.  If you use the same method to see these that you would use to see those 3D dot pattern images (uncrossing your eyes to focus on something behind the screen) you should see this image in 3D with the pink candle being closer to you. 

If you close one eye it removes 3D viewing.  We can still infer 3D info based on a couple of other visual clues.
Overlap cluesOverlap clues give us 3D info.  For example, since the pink candle at the left covers part of the blue candle we can infer that the pink candle is closer to us.
Size Clues Relative size clues can also give us some 3D info.  Larger objects are assumed to be closer than smaller objects. This only works if you have some pre-existing experience with the size of the objects you are looking at.
Focus clues can also give you some information about distance.  The muscles in your eye that focus the lens give you info on how far away something is, but this is limited to distances that are only a few meters away.
Astigmatism
An astigmatism is an eye problem that results from a the lens or cornea being out of shape or not symetrically shaped.