RE: SCOPE OBJECTIVE 40mm vs. 50mm ? ?
Mostly, its not about light gathering or whatever buzz word they use. Its about resolving small objects far away. In theory, a 50mm will resolve an image than a 40mm will. It is 36% more surface area.
For deer, moose, etc, I like 40-44mm. But groundhogs, and such, I like 50mm.