[Brightcolours]

Thanks for the reply. I understand your point about the fine point AF and have also found that the AF sensitive reduces with the smaller sampling window on the A7s. However, even with that setting, the light level in which AF can be acquired accurately on the a7s is still substantially lower than in DSLRs I have tried. I am basing my opinion here on a Pentax k-3 (which despite some other AF shortcomings has a class leading low-light AF sensitivity, plausibly rated to -3EV by the manufacturer ) and a Nikon D800. Even in very low light levels where you are not typically able to discern the scene in a DSLR viewfinder, let alone focus at all, the A7S AF works well albeit slowly - the DSLR need substantially more light to get any sort of lock. I have not used the A7rII so can't compare to the A7s directly, however I have used the A7ii and the low light AF sensitivity of the A7s is substantially better. I suspect the A7Rii performance must be more similar to the A7ii and not benefit from full sensor readout and higher S/N ratio as the A7s does. 

 

So in short I would say that in my experience the a7s not only goes down to DSLR levels but complete surpasses them in senstitivity and accuracy in low light - this was my main motivation for buying the camera as I was sick of inaccurate DSLR AF with fast lenses in low light.

Not every manufacturer's spec sheet is created equal then?

I got a 9 stop ND filter for my bday from a friend, and just now tried a little experiment before sleeping. It is night, and I only have a dim light on. EF 35mm f2 (old DC motor version) on my 6D, Hoya HMC ND400 mounted on it. Camera locks focus, at ISO 1600, f2.8 setting, 2.5 second metered exposure time in evaluative metering mode (kinda similar to matrix metering from Nikon).

Or ISO 12800, f2.8, 0.3 sec exposure time.

 

Indeed difficult to see anything through the OVF, only the lightest parts stand out from the dark. The 6D's live view with CD AF at ISO 12800 can't get that old CD motor lens to lock on focus. The 6D's PD AF option with live view clears the focus lock task with relative ease at around a second.