for SNA it must be CORRECTABLE to 20/20. Meaning that if you have 20/40 you can still fly as long as you wear glasses that correct your vision to 20/20. If you have some type of condition where you will never be able to see 20/20 even with glasses on this would disqualify you from SNA. PRK is for the people who have worse vision than 20/40, those people require PRK to improve their vision to be eligible for a pilot slot.
I hope this clears things up.