16-35mm lens problem?

1. Dark corners: Did you have a filter on? If yes, try not using the filter. You don't even have to take any pictures. If it's there, you can see it without turning the camera on. Did you use the right hood ? and put the hood on in correct position? Try without the hood to see if there is anything different

2. Black dots: more likely they are something sticked on your sensor. Do a sensor dust test

Did you have a lens hood/shade on?
If yes, you might not have turn that all the way and locks it.

No, I didn't use a filter. I use the hood comes with the lens. The two dark corners come and go. That's why I am thinking it may be related to moisture. thanks John.
Just saw Tom's reply. I didn't check the hood. That could be the cause. will check if it happens next time. thanks!
update: I've just managed to reproduce the problems (dark corners). It's the hood (when it is not tight all the way) causing the problem. Thanks John and Tommy!

The two diagonally opposed dark corners could very well be a lens hood that is not properly locked in the correct position. Presumably you are using the HB-23 hood? Make sure it is properly installed on the front bayonet mount and turned until you here it click and lock into place.

Do you see black dots in photos of blue sky or do you actually see black dots when looking directly into the lens? The first would be sensor dust. The second could be anything.

It's the lens hood, I have the same problem with some of my lenses. The moment the hood is not in place properly this vignetting problem occurs. Turn until it locks. Or send the lens to me, and wait for eternity....

Michael /Arthur,
The dark corners were caused by improperly positioning of the hood. I only seem to see black dots on the lens through the viewfinder. I don't see any block dots on the photos from the computer monitor. I'm wondering if these very small black dots were caused by lenspen which I've used for cleaning.? Although I've used lenspen for the other DX lens and it was fine.
thanks.

I'm glad it's solved. It would have driven me nuts to detect this precisesly in Venice! Anyway, spending some days in Venice with a D700 and the 16-35 must have been a nightmare. For the memory cards I mean.

Spots that you see in the viewfinder, but not in an image is 99% of the time dirt on your mirror.

... dirt on your mirror

Focusing screen. The mirror is not in the plane of focus, and dust/dirt on the mirror is not rendered in sharp focus in the viewfinder.