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The human eye-lens flexes to change focal length and thereby bring objects of various distance into focus. The magnification of the lens $M$ is always constant for an object with distance $d_o$ because the image distance $d_i$ is also constant as the thin lens formula for magnification implies: $$M = \frac{d_o}{d_i}$$ But this is only true if the image is in focus. If it is not, the image distance $d_i$ could change while the object distance $d_o$ remains the same and we would see the same object at a different magnification.
Q: Why does this never happen?

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  • $\begingroup$ Cross your eyes while observing something like your keyboard. Details of the image will appear to get smaller. $\endgroup$ – Hot Licks Mar 29 '18 at 0:30
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Eyes do loose their lock into the subject they have focused on after while, and the subject may look bigger/smaller, warped different hue and even twisted. One of the benefits of human's binocular vision is double checking of each eye's visual signal by the other.

The brain compensates, usually by blinking and refocusing. If the eyes are tasked with long precision work and strained beyond what they can handle, they react by tearing and frequent blinking and causing headache.

A large part of seeing is done in brain's visual cortex and it automatically censors images or parts of scenes that may look wrong or out of place.

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