I also asked this on StackOverflow but this is a better place to ask the question.
Suppose an object of 4cm is right in front of my eyes. I move it further away till it appears to me to be of 2cm. Is there a way to find out how much I moved it?
I tried using the concept of similar triangles, but, we need to have another length given to us to calculate the distance.
Another idea I thought was to use the focal length of the eye. But, couldn't find a way to use it.
EDIT: I think it would be more clear if I give an example. Suppose I close one of my eyes and put my thumb above the nose (the distance of the thumb from my eye is close to 0) and I measure the length of my thumb to be 4 cm. Now, as I slowly move it away from me, the length of the thumb from my eye appears to be smaller. Suppose this height from my eye becomes 2cm at some distance x (from my eye). I want to calculate x.