as far as i know, there are no limits as to how bad your eyesight can be
That may be true where you are, but in Europe there is a limit on how much correction is permitted. Extract below from the current CAA document:
These are the JAR Class 1 visual standards as applied by the UK CAA.
Distance Vision
Your visual acuity (measured by your ability to see, in this case, lines of letters on a chart at 6 metres) must be at least 6/9 in each eye separately and 6/6 using both eyes together, with or without glasses or contact lenses (correction). If you need correction, the refractive error (the amount of correction) must not exceed +5.00 dioptres of long sight or -6.00 dioptres of short sight. This is in the most ametropic meridian (taking into account any astigmatism). Astigmatism must not exceed 2.00 dioptres. The difference in correction between each eye (anisometropia) must not be more than 2.00 dioptres. Your optometrist will be able to explain these terms. If your refraction exceeds the limits stated above, please contact the CAA medical department for further advice.
Near Vision
On the standard near vision eye chart you must be able to read the N5 print between 30 and 50 cm and the N14 print at 100 cm, with or without correction.