The 6D's 35.8 x 23.9mm CMOS image sensor might be a bit smaller than the imaging chip in the Canon 5D Mark III, but its individual photosites are bigger since it has slightly less resolution.
Finally, the Canon EOS 6D Mark II sports a vari-angle touchscreen LCD, which is great for photographing pets. You’re able to adjust the screen and easily capture lower angles without hurting your back in the process! 2. Canon EOS 5D Mark IV. Image from Amazon. Canon EOS 5D Mark IV.
I wouldn't bother with the 5D II and its fixed pattern banding, the 5D III is good but doesn't have the 5D's look, and the 6D has the best image quality by far of those three but it's not the same look as the 5D, only the Mark II N has that look I know you want.
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