Teleconverters

can you use a teleconverter with a nikon D80 with a 18-200 VR lens and are the results ok

Not with Nikon teleconverters; they won't physically mate. You can use some 3rd party teleconverters with that lens. Results are OK only if you have very low standards.

Teleconverters were designed for use with fast telephoto lenses, not wide zooms. Just say NO to teleconverter abuse!

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RB
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There are ways to physically make it happen, but the image quality and focus suffer so much as to make it pointless to do it.
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Not recommended. The maximum actual aperture of that zoom lens without a teleconverter is already f/5.6. And as a rule autofocus systems have trouble doing their job with effective apertures smaller (or f-numbers higher) than f/5.6. With a 1.4x teleconverter, a 200mm f/5.6 lens becomes a 280mm f/8 lens; with a 2x teleconverter, the same 200mm f/5.6 lens becomes a 400mm f/11 lens! (All of those higher effective f-numbers are at wide open.) The f-stop loss will create problems with the autofocusing, and even manual focus is a challenge due to the substantially darkened viewfinder.

If you want a longer reach than 200mm (which effectively becomes 300mm on a 1.5x crop sensor), you're way better off getting a new lens such as the AF-S 70-300mm f/4.5-5.6 IF-ED VR lens.

Thanks for all your advice