Opticians/optometrists - have you ever seen somone's impaired vision cure itself?

Depends: If the impaired vision is due to an external process such as infection or allergy, it may 'correct' itself. Visual impairment from refractive error rarely cure themselves although if small, spontaneous changes in internal eye structures such as those with aging may seemingly correct the vision. This is not something you can count upon. Advice from the real eyemd - ophthalmologist.