[Ayoh]

I am surprised by your comments and have to disagree. With the A7s you mentioned, the ability to focus accurately in low light is unprecedented. AF works down to quite low levels and after that accurate MF is possible in levels where you can barely see with the naked eye. Compared to any DSLR i tried, the accuracy of AF and MF is much better - especially with fast primes in low light. DSLR make too many focus errors with fast lenses in low light. There is an issue with speed which can be slower than DSLRs (particularly in AFC) but the accuracy and light level operating window is much better

 

If you need ultra-high ISO then mirrorless is not for you.

That's not because of sensor noise (the Sony A7s would rule them all here) but in terms of AF.

Contrast detection AF is much more accurate than phase-detection AF on DSLRs but this advantage fades towards very low light.

But that depends on the amount of light we are talking about here.

 

The question of the day is however ... why do you want to upgrade at all ? :-)