Need Help, light gets darker at long end of lens

[tsambis]

Sorry if this has been covered somewhere else.
Here's my problem:
When I push in to the longest part of the lens, it seems like I lose a few stops in light.
Auto Iris is off.
Zebra bars confirm that there is less luminance when I push in on an object that was clipping while the lens was wide.

Any idea what this might be?
Thanks,

 

[DCSensui]

The max aperature of the zoom lens is smaller at the long end than at the wide end. It's in the documentation.

It's a problem only when trying to shoot wide open. To avoid it, Set your aperature at the telephoto end first (any setting except "open"), then it should remain constant throughout the zoom range.

 

[tsambis]

many thanks.