Guide:Measuring with differentials

It is not uncommon to have trouble with measuring accurately, especially for individuals who have -4 or more diopters of myopia, or a fair bit of cylinder requirement. The reason being that there is a much smaller margin for error in a range where one cm or less makes a difference of a quarter diopter. Also for those with higher cyl needs find that directional blur adds a lot of challenge . The best way to overcome either issue is to measure with differentials. Chances are you know your differential correction without even having to look it up so substitute your correction and edge of blur into the example.

Example

Suppose you are wearing a -4 differential in both eyes and you find your edge of blur to be 68cm. Go to the calculator, and enter 68cm. The result is -1.5. The -4 you started with combines with the -1.5 you just determined you still require to indicate your true myopia to be -5.5.

Suppose you have OD/OS -4.25/-3.25, and find your edge of blur to be 68cm.

Go to the calculator, and enter in 68cm. The result is -1.50. Add this value to both of your eyes (be careful with negative numbers).

$OD\ (right\ eye):-4.25+(-1.50)=-5.75$

$OS\ (left\ eye):-3.25+(-1.50)=-4.75$

So, your true myopia values are OD/OS -5.75/-4.75. Simple!

See Also

Guide:How to measure your eyesight