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The prescriptions can reach extremely high, although I am not sure what the exact maximum is. Spectacles with a very high prescription will not always prevent blindness though. For example, someone with a prescription of -30 diopters can still only have a corrected visual acuity of less than 20/200(which is considered as legal blindness). The high prescription is over -6.00D. It is not possible it can reach until -50.00