Ok it must be due to the larger exit pupil. I am looking at some Pentax DCF HRII in a 10x42 with a relative brightness of 17.6 or there is a 8X42 with a brightness of 27.6. One more question. What exactly does this number refer to or mean? Since I usually hunt in more open country I think it would be best I go with the 10X so I guess you have to give a little to go one way or the other.