Canon-Chas wrote in post #17360409
I now have both the 7D MK2 and 1D MK 4 using a 300mm f/2.8 MK II with or without 1.4X and 2x TC MK III.
Weather is bad, so might get out tomorrow.
I guess it depends on what you photograph, and the light available
... or how big your subject is, and how close you can get to it. I have seen absolutely zilch for evidence that with the same lens, aperture, shutter speed, lighting, subject and subject distance, that the 1D4 gives better IQ when cropped to APS-C size. People's subjective assessment that "the 1D4 has better IQ" is irrelevant, if their assessment is based on 100% pixel views, or resizing to the monitor resolution with "nearest Neighbor".
I have a 6D and a 7D2. When I had a 7D, I brought the 6D along also when shooting birds when the light was low; the loss of subject resolution in FLLP (focal-length-limited photography) was negated by the lower noise at high ISOs. With the 7D2, bringing the 6D along is totally unnecessary, as the 7D2 is better than the 6D for FLLP at low ISOs and in resolution, and about the same for high ISO noise. Not a plus to be found on the 6D, unless I use the entire frame.