Description : How to turn dioptres into focal length?
Last Answer : You’ve got it right. The dioptres = 1/focal length, so focal length = 1/dioptres. e.g. 3.3 dioptres = 1/(3.3) = 0.30303m focal length -2 dioptres = 1/(-2) = -0.5m focal length http://medical-dictionary.thefreedictionary.com/Dioptres for more info.