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Vision prescriptions

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F1fan | 00:12 Wed 16th Aug 2006 | Body & Soul
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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!
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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
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Thanks very much,that's a pretty comprehensive answer!

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