Andrew M - here is the text from JAR FCL3:
JAR–FCL 3.220 Visual requirements
(a) Distant visual acuity. Distant visual acuity, with or without correction, shall be 6/9 or better in each eye separately and binocular visual acuity shall be 6/6 or better (see JAR–FCL 3.220(h) below). No limits apply to uncorrected visual acuity.
(b) Refractive errors. Refractive error is defined as the deviation from emmetropia measured in dioptres in the most ametropic meridian. Refraction shall be measured by standard methods (see paragraph 1 Appendix 13 to Subpart B). Applicants shall be considered fit with respect to refractive errors if they meet the following requirements:
(1) At the initial examination the refractive error shall not exceed ±3 dioptres.
(2) At revalidation or renewal examinations, an applicant experienced to the satisfaction of the Authority with refractive errors up to +3/-5 dioptres and with a history of stable vision may be considered fit by the AMS (see paragraph 2 Appendix 13 to Subpart B).
(3) In an applicant with a refractive error with an astigmatic component, the astigmatism shall not exceed 2·0 dioptres.
(4) The difference in refractive error between the two eyes (anisometropia) shall not exceed 2·0 dioptres.
This is a non-medical recitation of the rules. I shall leave it to the assembled community to add the weight of personal experience and medical knowledge!!