John from PA wrote in post #17013116
With some lenses designed for APS-C the camera mirror of a full frame body will hit the back of the lens and damage the mirror and maybe the lens. I suggest you use extreme caution in testing lenses moving forward. There is generally no second chance; one simple shutter click can damage the mirror and/or lens. The 18-55mm EF-S kit lens commonly sold with the T4i should not be used with a 6D.
The damage is only possible with some Canon EF-S lenses as their design allows for the rear to protrude further into the mirror box, usually at the short end of a zoom. That is why they specifically designed the different EF-S mount, so that such lenses cannot be fitted to FF cameras by mistake.
Third party manufacturers, such as Tamron and Sigma, do not use such a short back focus design and so their APS-C only lenses do not protrude far enough back to risk hitting the mirror. So, they have used the EF mount as it is safe to do so. The only problem is the extreme vignetting from the small image dircle.
The reason for the vignetting is obvious, less so is why "the grainies" set in with a 2x converter. Using a TC in itself shouldn't cause any noise or grain (I am assuming the OP is referring to getting a noisier image) however as it knocks two stops off the maximum aperture, the camera (if it is on auto ISO) is possibly bumping the ISO to a higher level, which may be causing noise (although the 6D is so good at noise control, I would imagine that it should be able to shoot two stops higher ISO than a T4i without significantly more obvious noise).