CBM SC,

Honestly that all depends on the Make and Model. If you are talking about equal scopes, the larger the diamater the more light will be transmitted. But is it worth the extra weight and higher rings? My opinion No. Comparing a 40mm to a 50mm, you will not add that much more time to your hunting. Also, when looking at light transmission, another thing to look at is the exit pupil. To calculated the exit pupil, you will take the objective size divided by the power setting. Example: a 3-9x40 scope, at 3 power your exit pupil is 13.3mm where at 9 power it is 4.4mm. With a 3-9x50, on 3 power it is 16.6mm and at 9 power is it 5.5mm. A normal and healthy eye can only except 7.0mm of light, so anything more than that is unuseable. Here is Michigan, I use a 3-9x40 and I have no problem seeing the deer or crosshairs during normal shooting hours. I am not familiar with the Zeiss line, but have heard that they are very bright. So actually your 3-9x42 maybe brighter than alot of your 3-9x50's. Hopefully this answers your questions.

Jon Jackoviak
The Optic Zone
www.theopticzone.com
[email protected]