Originally posted by JimB: Besides the power issue,the 8X42's will gather light about 20% better than 10X42's,if all else is equal.Divide 42mm by 8 power and you get a factor of 5.This is the maximum that your eye can use.Divide 42mm by 10 power and get a factor of 4.Less light gathering.If the 10 powers were actually 50mm,light gathering would be about equal.