The D800 and A7R are not appropriate for sports photography... the 1D X is.
The D800 and A7R both have a top burst speed of between 4 and 6 frames per second.
Also keep in mind... not everyone is nuts about these high res sensors. There are laws of physics which govern the REAL maximum resolution you can have (regardless of glass or sensor technology -- you cannot get around these limits). These are established by diffraction limits based on the wave nature of light.
The D800 is a 36MP camera at or below f/5.6. It is not diffration limited. If you shoot at f/8 it starts to become diffraction limited, but the Airy disk is only slightly overlapping onto adjacent pixels. By f/11 it is fully diffraction limited -- the overlap of the Airy disk is substantial. The camera will record 36MP files at any f-stop... but it's not possible to separate detail between adjacent pixels at full resolution anymore.
Keep in mind... diffraction is based on laws of physics. Lens quality, camera quality, sensor quality, are not able to bypass these no more than we can violate the speed of light or ignore gravity. The math for the calculations assume "perfect" optics (optical flaws are not considered.)
Light does not focus to a point. Due to the wave nature of light, it focuses to something called an "Airy Disk" as described in papers by astronomy George Airy back in 1835.
In order to not be diffraction limited, the physical size of the Airy disk has to be smaller then the physcal size of a single pixel (usually measured in microns).
This implies that as the Airy disk gets larger (something you cannot do anything about -- that's a law of physics) you would need a larger "pixel" (something you can control). But if you increase the size of the individual pixels but do not increase the total sensor real-estate, then the maximum number of pixels that will fit on the sensor will be smaller. You'd have to physically make a bigger sensor with bigger photosites in order to avoid the problem.
This means that if you REALLY want a high-resolution sensor, you have to go to a physically larger sensor format such as a medium format camera (like a Hasselblad -- but those are very expensive.)
You can read more about diffraction
here: http://en.wikipedia.org/wiki/Diffraction
and here: http://en.wikipedia.org/wiki/Airy_disk
But I think on of the more applicable treatments is here: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
And that's why I've never been obsessed with extremely high res sensors. My "other" hobby is astronomy... so this is a problem we encounter regularly. It's the reason I can't just stick a really high power eyepiece in a small telescope and see objects in equisite detail. If you want more detail, you need a MUCH MUCH MUCH larger telescope (which is why astronomers are obsessed with building these massive telescopes.)
Related to this, is something called Dawes' Limit. This says that in addition to the diffraction limits, there's also a limit on how much detail that can be resolved based on the physical diameter of the lens.
I don't want to mislead... it's not that high res sensors are not useful... they are. But only if you shoot at low focal ratios where diffraction isn't a problem.
Tim Campbell
5D III, 5D IV, 60Da
