All other things being equal, pupil size is far and away the biggest
determiner of unaided acuity between two myopes.  Just like a camera, if
you set the focus at a particular distance, changing the f-stop will
profoundly affect the photo.  That's why cheap cameras with tiny
apertures don't really need any focusing mechanism, while expensive ones
with large lenses and variable f-stops need such fine focusing.  In the
extreme example, take the -3 patient and have them look through a
pinhole.  They will be close to 20/20 without any lens at all.  Take the
pinhole away and they'd be lucky to get 20/200.

w.stacy, o.d.

Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision?  For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other.  What factors
influence this?