What could cause short-sightedness in one eye but not another?

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Thank you for your query.

Any significant difference in the power of sight between the eyes may cause amblyopia. In a common language, it is called lazy eye. The effect of amblyopia is in the brain as it gets used to seeing a blurry vision. Later if a person uses glasses, the brain will poorly respond to the glasses or not respond at all with no improvement in vision. Amblyopia is generally treatable until the age of 17 years. However, the treatment takes time and patience and is usually done by a patchy, better-seeing eye to force the brain to use the weaker eye. Please do not hesitate to ask if you have any further questions. Thank you.