My mind has replaced my eye's function: everything I see transforms into your image, rendering my vision unreliable.
Shakespeare presents a startling neurological crisis: the eye 'part[s] his function' and becomes 'partly blind' while simultaneously appearing to see. This isn't mere distraction but a fracture in perception itself. The eye delivers nothing to the heart; the mind intercepts all sensory data and replaces it with your image. The 'form' of external reality—bird, flower, mountain, day—becomes irrelevant.
The concluding couplet contains the sonnet's deepest irony: the speaker's mind is 'most true' precisely because it is also 'untrue.' By being completely flooded with the young man's image, the mind achieves absolute fidelity to obsession—a perverse kind of constancy. Truth and falsehood collapse into each other when love has hijacked cognition entirely.
Like checking your phone every five minutes and seeing only one person's messages reflected everywhere—your dating app becomes their profile, your Instagram feed reshapes itself into their photos, your brain runs this filter so compulsively you stop trusting what you actually see.