« Last post by 3kramd5 on April 22, 2014, 10:57:18 PM »
Regarding AF point-linked spot metering for the 5DIII, while the AF systems are nearly the same as you state, the metering systems are vastly different. Here are the 61 AF points superimposed on the 5DIII's 63 zone iFCL metering grid:
The resolution of the 5DIII's metering sensor simply may not be high enough to support spot metering with the AF points, whereas the 100,000 pixel metering sensor of the 1D X can do so. Even when the 1D X's metering sensor reverts to zone metering (in very dim light or for flash exposure metering), it's divided into 252 zones - 4 times the density of the 5DIII's metering sensor.
I can't say for sure that those tecnhical limitations are absolute, but you might consider the possibility that there are technical reasons for those features being available on the 1D X but not on the 5DIII. After all, they did add f/8 AF to the 5DIII.
How dare you suggest there is a fundamental reason that the 5D3 and 1Dx don't operate the same way?
Personally, I imagine that if the spot meter works at the center, one could use whichever of the 63 segments corresponds to the AF point in question. Given how large the AF sensors are relative to the total frame, in all likelihood each AF point falls on multiple segments with the 5D; and it's almost certainly the case with the 1Dx. I would guess that the 1Dx uses an average reading of all the appropriate segments rather than a single segment. Given the size of the spot meter indicator in the VF, I further guess that the center spot metering uses multiple readings.
The 5D3 could probably do the same thing, but it may negatively affect performance given the substantially lower processing power relative to the 1Dx.