Is it possible that a person is operated for wrong power ? For example if a person power is  -4D, but the optometrist diagnosed it -4.5 D during subjective refraction tests. Then what is the possibility that the person will be operated for -4.5 D instead of -4 D?. Aren't there any machine which calculates eye power by itself? ( Because human error is not involved in machines!). I am scared if doctor operate me for wrong or true eye power and if later I have to regret. I heard that auto refractometer calculates eye power precisely, then what should we do to avoid this human error?