Did is no direct translation. One (-5) is a measurement of your refraction and Snellen is a measurement of acuity.
A -5 would be less than the big letter "E" which is 20/400.
Thank you very much - I did actually do what seemed to be the Snellen test although the figures seemed different in Spain than in England. I could see the big letter E, despite being -5 in both eyes and was told I had 25% vision without correction, or 45/150. With correction I was quoted 100% vision and 6/20 (which seemed slightly confusing - surely 6/6 would be 100%?)
Anyway, I appreciate your help, if the above information makes sense to you at all I would be interested if anyone could explain it!