18-200 and vignetting with Cokin P holder

[21tones]

I'm using a Cokin P Holder on top of a Canon UV filter on my 18-200 zoom lens.
I'm puzzled why I'm getting vignetting at 18mm when using a Cokin polariser.
I've used the Cokin setup and the same Canon UV filter on my old FD 20-35 zoom in the past and was able to use it down to 24mm before vignetting started. 18mm on the crop format is equivalent to 29mm. It doesn't make sense!
Is it somethng to do with the design of the 18-200?
Of course I could remove the UV filter as it doesn't vignette then, but I'm always concerned about not having some protection on the front of the lens. I sometimes bang the UV filter glass when taking the Cokin adapter on and off if I am rushing to get a shot in changing light. I don't want to be doing that to the front of the lens!
Has anyone tried the slim UV filters like the Hoya Digital Pro 1 on this lens with the same Cokin setup? Does it vignette at 18mm?
What's even more confusing is that I can use a Hoya Circular Polariser on the front of my Sigma 10-20 (with no UV filter on) and then put the Cokin holder on the front of that and not get vignetting until 14mm! The Hoya is the same thickness as the Canon UV.
Do others encounter these problems with the 18-200, or is mine atypical in some way?

With the filter thread on this lens being plastic I wonder how robust it is over time?
I've only just got mine and I put my Cokin filter holder on and off constantly.
Has anyone who has had an 18-200 for a few years found problems with the filter thread e.g. getting worn?
I'm toying with the idea of leaving the Cokin adapter in place all the time and using a Cokin ring cap in lieu of the Canon lens cap.
But then I wonder whether the pressure of putting the filter holder constantly on and off the adapter ring, when its attached to the lens, will cause problems on the lens filter thread.
Any experience or thoughts about this?
Thanks

[K-amps]

I am no expert, but perhaps vignetting depends on the width of the front opening of the lens where as crop is the final image on the sensor. The width/focus of the beam will change in size from the front to the back of the lens. This may be true more so in an EF-S lens, OTOH if it were an EF lens, I'd be more concerned.

I get vignetting below 20mm on my Cokin P, on a 24-105 F4.

Is your holder 1 or 2 filter slotted? I know guys who have hacked off the last (protruding) slot making it only a single slot holder, but that gives them extra width. One guys said he could do 17mm after that... but I forget exactly. Perhaps a search of the forums will lead to some answers.

[21tones]

Thanks for your response.
I'd thought about trying to reduce the size of the filter holder.
Looks like its out with the hacksaw this weekend!

[wickidwombat]

Thanks for your response.
I'd thought about trying to reduce the size of the filter holder.
Looks like its out with the hacksaw this weekend!

yep, I did this to a cheap plastic ebay one to stop vignetting on my 16-35 on FF it fixed the problem

[K-amps]

Thanks for your response.
I'd thought about trying to reduce the size of the filter holder.
Looks like its out with the hacksaw this weekend!

yep, I did this to a cheap plastic ebay one to stop vignetting on my 16-35 on FF it fixed the problem

WW: Did you use an 82mm holder or an step-down adapter ?