In camera tracking how do i figure out the settings for the cannon 6d In camera tracking how do i figure out the settings for a cannon 6d. I know the sensor width it 35.8 but I'm unsure about what to use for the other settings. Especially pixel width and optical center.

Thanks
Optical center should be the center of your image height/2 , width/2 ,  unless you have cropped the image. Then it should be the original center of your image. There is button for this labeled center. This is relevant to accounting for lens distortion.  The 

Pixel aspect ratio should be one unless you used something like an anamorphic lens, which stretches the pixels when the image distortion is countered in post.