Camera Labs

hi i have a nikon d80 and have purchased a sigma 18-200 lense, i am trying to find a 2x converter that will work with the camera and lense,any help would be appreciated

Hello manorjim, and welcome to the friendly Camera Labs forum!
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There is a 2.0x tele converter EX DG from Sigma that should work
...in principle
But I wouldn't expect autofocus to work at longer focal lengths as you get an effective aperture of f/11
And image quality deteriorates so much that it could be easier to crop/enlarge the original image (w/o converter) in postprocessing

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Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews, My Pictures, My Photography Blog
D800+assorted lenses

The Sigma 2x teleconverter is not listed as compatible with their 18-200. In general, teleconverters don't like lenses capable of wider angles, which often have a rear element placed far back. The Sigma teleconverters have quite a protrusion, so I fear that might physically conflict. Generic teleconverters like those from Kenko don't protrude so much so have a higher chance of fitting.

As Thomas says you will likely lose AF ability and it will come with degradation in image quality. If you need longer focal lengths, ideally look at lenses which cover those naturally.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.

Oh right, popo! Thanks for the hint.
Here is the compatibility table for the Sigma converters and the 18-200 is not listed

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Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews, My Pictures, My Photography Blog
D800+assorted lenses