In our tests with the 1D X, we found its image quality to be well-suited for the type of work it's expected to do, delivering high resolution images with low image noise. It scored 2200 x 2000 LPH (vertical and horizontal) on our resolution chart, which isn't game-changing but is certainly very good.
ISO performance is another area where the 1D X scores well. Images remain remarkably clean up until ISO 3200, where you can first see noise start to interfere with image detail, albeit at a negligible level. It's at ISO 8000 where image details start to smudge, and at ISO 20,000 to 25,600 it's when noise starts to show, especially so when we reach ISO 32,000. We'd actually be comfortable shooting at this level, and up to ISO 51,200 if necessary. Image noise is mostly luminance noise with very little color interference. The image above was shot at ISO 51,200, and it's perfectly acceptable for us if we were just going to share it on the internet.