This is the rule in question, (listed under the Death Note Wiki as "How to Use: XXI"):
"Those with the eye power of the god of death will have the eyesight of over 3.6 in the human measurement, regardless of their original eyesight."
So, what are the numbers referring to? 3.6 what? I googled Visual Acuity, and I'm guessing those with death god eyes can see over 3 and a half times further than normal humans? 3.6 times to be exact.
I never read the manga, or saw the Japanese version, so I ask this question to confirm it with perhaps something someone read.