Originally Posted by Richard Lane
Correct - no 1-series body before the 1D X has had a dual cross point, although the 7D has one...as do the 40D/50D/60D. Indeed, many of the 7D's features were brought to the series with the 1D X, or improved - you mentioned one earlier, Spot AF. It was interesting that Spot AF was available on the 1D IV, but only with a supertele lens attached, whereas on the 7D you can use Spot AF with any lens, which will be the case with the 1D X.
True - but there's a trade off. Seems to be the story with the 1D X AF system. Yes, the 1D X brings 5 f/2.8 high-precision dual cross points to the 1-series...but it eliminates the ability to AF at f/8 with the center AF point, and while the center point of previous 1-series bodies operated in high-precision mode with an f/4 lens (like Rick's 500/4), the 1D X requires an f/2.8 lens for high-precision functionality with those 5 points.
Technically, yes...but there's an additional implication that should be discussed, and it's yet another of those trade-offs that make it tough to say which is better. An f/2.8-sensitive sensor line has a wider baseline, meaning it can achieve a more accurate focus than a sensor with an f/4 baseline (but of course, the f/4 sensor works with more lenses).
With an f/2.8 lens, the 1D IV has 39 very accurate f/2.8-sensitive cross-type points, with the center one as high-precision. With the same f/2.8 lens on a 1D X, you have 5 even more accurate f/2.8 dual cross-type points, all of them high-precision, but 20 slightly less accurate f/4-sensitive cross-type points, and 16 slightly even less accurate f/5.6 cross type points. Also, on the 1D IV the f/2.8 points are spread across the frame, while on the 1D X the most accurate points are right in the middle. IMO, that means with an f/2.8 lens, the 1D IV is the winner - 39 spread out highly accurate cross-type points vs. 41 less spread out and mostly less accurate cross-type points.
With an f/4 lens, the 1D IV has just one cross-type point, although it operates at high precision. That same lens on a 1D X will have 20 equally accurate f/4-sensitive cross-type points at the sides, and 21 slightly less accurate f/5.6-sensitive cross-type points in the center block, for a total of 41 cross-type points vs. just one. So, the 1D X is the clear winner with an f/4 lens.
Here's where the exceptions on that list of Rick's from the 1D IV manual matter, since you have the 300/2.8 + 1.4x that is one of the exceptions. On the 1D IV, it will behave like an f/2.8 lens, as will the very commonly used 24-105mm and 17-40mm, and based on the criteria above, those lenses will be better on the 1D IV than on the 1D X. I honestly don't know if you'll get more accurate focus when using an exception-list f/4 lens on an f/2.8 sensor, compared to that lens on an f/4 sensor. I suspect not, but it will still be acting as a cross-type sensor - and the cross-type geometry is probably more important in overall performance than the aperture baseline, because in most situations, line orientation determines whether an AF point works at all, vs. subtle difference in accuracy.
So, based on the raw specs of the AF sensors, the lens(s) you have will impact which 1-series AF system is actually better for you (although that will be impacted by the benefit imparted by contributions from the metering sensor, no way to really quantify that). For example, I think you'd be better off with the 1D IV for your 300/2.8 + 1.4x, while Rick would be better off with the 1D X for his 500/4, unless he wanted to use it with the 2x TC, in which case he'd be better off with the 1D IV.
Head spinning yet? Mine is.....
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