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I had been using the Tamron 18-270mm lens with my old canon rebel camera and it worked just fine; but now I have the canon 6D camera and when I use it with the same lens it comes out with a complete and dark vignetting. I don't understand why this is happening! Is there something I can do to fix this?

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    Having upgraded to full-frame, you deserve to upgrade to full-frame lens. You wouldn't want anything of that poor quality anyway with a high-end camera, otherwise upgrading would be pointless. – Itai Jul 14 '16 at 14:22
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From the "Recommended For" tab of the Tamron web page for that lens:

Tamron Di-II lenses are engineered expressly for digital SLR cameras with image sensors commonly referred to as APS-C, measuring approximately 24mm x 16mm.

This means the image circle is sized for the smaller APS-C sensor, and is too small for a full-frame camera like the 6D. This is true of all lenses designed specifically for APS-C, from all manufacturers.

Is there something I can do to fix this?

Unfortunately, no. You will need to replace it with a similar lens designed for a full-frame sensor.

As @Itai pointed out in a comment, even if you managed to find a hack using a teleconverter or other adapter, that lens is not up to the quality and resolution of the sensor. Images taken with that lens on a 6D will never be as good as when using an appropriate lens.

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That lens is designed to be used for APS-C sized sensors with the Canon EF-S mount. It projects a smaller image circle than is needed to cover the larger full frame sensor of your 6D. Most third party APS-C lenses do not include the additional tab that prevents them from being mounted on a FF Canon camera that uses the EF mount. That is why you can even mount it on the 6D.

Most third party APS-C lenses also don't use the (available) possibility of allowing lens elements of APS-C only lenses to protrude past the mounting flange into the light box of Canon APS-C cameras. They probably don't do it because they also use the same basic design for the Nikon, Sony, Pentax, etc. versions of the same lens model. Some of Canon's own EF-S lenses do exploit this possibility at wider angles and certain focus positions.

Is there something I can do to fix this?

Theoretically a 1.4X Teleconverter (not a Canon extender, which would not fit that lens at all) might expand the image circle just barely enough to cover the FF sensor. A 2X TC certainly would. But the reduction in maximum aperture, AF speed (if it will AF at all), and image quality with either would probably not be worth it.

For more about the difficulties of using a teleconverter with a variable aperture zoom lens, please see Are there any Teleconverters/Extenders which will work with a Canon Rebel EOS T3 and 55-250mm f/4-5.6 lens?

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