He's comparing an optometrist to a storefront eye exam. I've had one of those storefront eye exams once when traveling--better than nothing but the off by near

He's comparing an optometrist to a storefront eye exam. I've had one of those storefront eye exams once when traveling--better than nothing but the off by nearly a diopter. These days I'm much more sensitive, an error like that would leave me in a fair amount of pain. A quarter of a diopter change (my vision is nothing like stable) sends me running back to the optometrist I trust (I've had others not get it accurate enough.) On the other hand, I've known my wife to be half a diopter off and not think the new lenses were any better. (Replacing a several-year-old damaged pair.)