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This has been on my mind for a while. We know that the size of an object changes with perspective like when we move the object towards or away from our eye, it becomes bigger or smaller, respectively right?

But, say I have an object with an actual height of exactly 3 inches, measured from a ruler. If my eyes have built-in ruler system and I place the object directly in front of my eye, it would measure an infinite height. But when I place the object somewhere far, the object would appear very small, noticeably smaller than the actual size.

So how far should an object be from the eye/camera/etc so that the viewed height is exactly the same as the actual measured height? Please enlighten this poor soul, thank you! :D

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  • $\begingroup$ Without knowing something about the "built in ruler system" in your supposed eye, I cannot answer this question. Perhaps you cannot do so because you, too, do not have a clear notion of what this "built in ruler" actually is. One way to measure is to look at the height of the image formed on the retina (which for anything you look at is probably between 0 and 15mm or so). But I have a feeling that's not what you're talking about. $\endgroup$ – John Hughes Oct 22 '16 at 17:41
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It depends on the focal length of the eye lens and the built-in ruler system your eye has. Placing an object at 2*focal length forms the image of the same size.

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