Visual acuity question (involving eye number)?

Question

I understand the 20/20, 20/40, etc. numbers given from a Snellen chart, but I'm not entirely sure what the + or - numbers mean. I read that they mean the power of the lens required. So, does that mean that the two numbering systems are interchangeable? Is there a definite correlation between the two?

Thanks in advance.

Answer 1

If you mean like +2.00 or -3.50, then yes, those are the powers of the lenses in diopters (inverse meters).  A + number indicates that the lens is convex, converges light, and corrects farsightedness.  A - lens is concave, diverges light, and corrects nearsightedness.  The higher the number, the stronger the prescription.

There isn't really a correlation between this system and the Snellen numbers (the 20/xx).  There's an estimate that each line on the Snellen chart is about a 0.25 D change, but it's not totally reliable, especially with astigmatisms or higher prescriptions.  The only accurate way to determine your visual acuity is by actually looking at the chart.  Hope that helps.  :)