Vision prescriptions

[F1fan]

Can anyone tell me what the prescription 6/9,6,12 is in decimal form please, as in -2.00,-2.25 etc? Thanks in advance!

[kempie]

The Snellen fraction (6/9) used to describe unaided visual acuity is not a completely accurate predictor of refractive error; too many other factors influence unaided acuity.

As a rough guess, if myopia is known to be the cause of the reduced acuity and if there is not also astigmatism present, then unaided acuity of 6/9 would correspond to somewhere around -0.50 Dioptre.

This webpage offers approximate estimates for prescriptions based on Snellen fractions but because it is a US site it uses the non-metric "20" scale where:

20/30 = 6/9
20/20 = 6/6
20/40 = 6/12

[F1fan]

Thanks very much,that's a pretty comprehensive answer!