Not trying to jump in for Fast Glass, but also to clarify if my assumption was incorrect. I took the question to be a comparison of the PDAF through the viewfinder and DPAF using liveview both on the 1DXIII.

Overall, my key takeaway would be in line with Joel's. I am not having any significant issue with the AF on the R5. The AF definitely performs best with targets within a certain range and with plenty of light, but that is something that is true with all AF systems.

Quote Originally Posted by Joel Eade View Post
One possible difference is pixel size where the 1DXIII pixels are bigger and I think it is well established that bigger pixels tend to perform better in low light in terms of signal to noise ratio. Not sure if this includes DPAF performance.
I have thought about posting on this topic to see what others think. But, I am not sure this is true anymore. I see it referenced all the time, so I may be incorrect. But, after microlenses were first introduced and optimized, and the architecture size of the sensor chips decreased, I think those steps have largely taken away the pixel size issues. Before light either fell into the pixel well or hit the physical pixel well wall, essentially being lost. For the same side wall thickness, the more pixels you have, the ratio of pixel well area to top of side wall area decreases, so more light is lost bouncing off the top of the sidewalls of the pixel wells. But, microlenses act as funnels directing more light into the pixel well. Then, Canon switched to a smaller architecture, that also decreased the thickness of their sidewalls.

I could easily be wrong. But, I have also played with real world ramifications. What I can say is that I do not see much of a correlation between DR performance and pixel density, but I see a very strong correlation between DR and sensor size.

Another way to look at this, but Quantum Efficiency (efficiency of a sensor at capturing light) of the 5DII was 35%, 5DIII was 47%, and 5DIV was 54%. Move to the most modern sensors, the R6/1DXIII are 55% and 53% respectively (all taken from Photons to Photos website, but based on S/N ratio from DXO Mark). For the record, Sony's A7R III is 59% efficient. So, bottom line is modern sensors are capturing 53-59% of the light, a dramatic improvement over 35% of the 5DII. What is fun, still some headroom to improve into.

But, playing with pixel density, the 5DIV and 80D (same generation of sensor) have the same 54% efficiency. Even go back before that, the 5DSR had 49% efficiency compared to the 47% of the 5DIII. So, almost the same efficiency (5DSR released a year or two later, so perhaps some slight improvements).