Rifle Scopes S&B PMII 5-25x56 Question about 25x times and light transmission

[Bloodstriker]

I have a question for the users of the S&B PMII 5-25x56. When increasing magnification, I have noticed that the picture becomes dimmer once I pass around 20-22x. It's especially noticeable at nighttime.

Does anybody else experience this?

EDIT: I understand that with a scope, the higher the mag, the less light, but I'm just making sure.