Help, converting snellen acuities 10 feet chart viewed at 5 feet

[jeepj27]

Hello. I bought a 10 foot snellen chart and it's fine but we have to hang it 5 feet from the chair we are using? Can you help us convert the visual acuity it says on the 10 foot chart we got to represent us doing it 5 feet away??

[Darryl Meister]

At half the distance, the letters are effectively twice as large for a given line of visual acuity, so the visual acuity would be half as good (you would double the Snellen denominator). So the "20/20" line would actually be equal to "20/40," for instance.

Also keep in mind that there is 0.66 D of vergence from a chart located at 5 feet, which you would need to compensate for after any spectacle refraction.

Best regards,
Darryl

[jeepj27]

Thank you Darryl! I am wondering though, what about the numerator? As it stands, the chart has acuities such as 10/200, 10/100, 10/80, etc.... So would I also adjust the numerator in these examples to be 20/400, 20/200, 20/160?

Also regarding the vergence, if an ending refraction is say -1.00 -0.50 x 180, I would then add 0.66 to this making it -1.66 -0.50 x 180?

Forgive my naieveness with Optics - it's not always been a strong point!!

Thanks for any advice !!

[Darryl Meister]

Thank you Darryl! I am wondering though, what about the numerator? As it stands, the chart has acuities such as 10/200, 10/100, 10/80, etc.... So would I also adjust the numerator in these examples to be 20/400, 20/200, 20/160

No, you would just adjust the dominator for the shorter testing distance. The numerator would stay the same. If you want to convert the whole thing to the more common notation using 20 feet in the numerator, you would have to multiply the numerator and denominator by 2, after adjusting the denominator for the testing distance (so the numerator would get multiplied by 2 and the denominator would ultimately get multiplied by 4).

So, the 10/100 line would become 10/200 at 5 feet or 20/400. But this is all assuming that the chart is calibrated so that 10/10 is equal to perfect visual acuity at 10 feet. The letters of the 10/10 would therefore need to be about 4.4 mm tall. When in doubt, I would suggest calling the chart manufacturer to confirm your conversions.

Also regarding the vergence, if an ending refraction is say -1.00 -0.50 x 180, I would then add 0.66 to this making it -1.66 -0.50 x 180?

Yes, that is correct.

Best regards,
Darryl