Hi Ge-lee,
As far as I know that is quite normal, for all telefotos.
I find my Century starts vignetting about half way towards wide angle.
Mike

Wow, I thought I had bought a defective lense.

Sony 1.7X is the same way as the Century: Starts vignetting at half towards the wide.

Hello Dave....Did you like the Sony 1.7X lens?

Federico

Yes, Federico, I think the Sony 1.7X is a good lens. I have a review of this lens over at the DV Info forum's "Open DV Discussion". The post is entitled "Must Decide Today: Sony 1.7X Telephoto".
A couple of things I was most pleased about it is that it is very sharp and does not cut down on light transmission to the tape.
My biggest concern was the vignetting, which only appears at
the half-way zoomed wide position (and gets progressivly more pronounced as you zoom towards full wide). I wondered if the Century tele was this same way but now I see, from Mike Black's post, that the Century has the same vignetting.
I would recommend the Sony lens. Do note that the lens I have (and the one that was reviewed) is the "HG" (high grade) model.