[Ayoh]

The problem is that fine spot AF (which I am using all the time) is lost in very low light. In this case the camera is expanding the AF field to look at the "grand picture".

 

That is not really surprising - the sensor noise is very high in such a situation and this has impacts. The same is true for DSLRs but the AF photodiodes are MUCH bigger thus not quite as prone to noise.

 

I haven't used the A7S (II) - it's probably much better than on the A7R II - but I still doubt that it goes down to DSLR levels here based on spot AF. Feel free to correct me though.

 

As mentioned we would have to define what "low light" means. I am talking about -say- 10 candles in the living room here and focusing on a person with fine spot AF.

 

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.