Originally posted by A.Styrlander Thank you guys for giving me your thoughts on this, much appreciated.
It's a tough decision for sure, I'm not really contemplating moving from crop-sensor to FF anytime soon, why? Mostly because I'm very pleased with my results for most types of shooting and I already own many of the best aps-c lenses out there for K-mount. I also enjoy the much smaller size of zoom lenses in particular for crop systems rather than those for FF. So aps-c will stay as my main system for many years to come I believe.
But if I should buy a K1 as a speciality body for lowlight shooting I will see much better results of course than my current K3. But as I'm also a lot into cityscapes (shooting old factories and city skylines) the dynamic range of the 645Z really appeals to me, In these types of shots I always use a tripod of course. Also if I get the 645Z I will get a more competent camera for macro, portraits and studio work which I'm looking to get into more in the near future. I understand that a A7IIs for example would be best suitable for those documentary street night shots, but I won't get the benefits for my other types of photography that I would with a 645Z.
However I specifically asked for the 645Zs capability in lowlight situations handheld and maybe its a bit unsuitable for this ofcourse but still will be a huge step up than my K3 I guess.
I guess the thing is that while the 645Z does have impressive sensitivity out to 12,800, you're currently limited to f/2.8 lenses at the fastest, unless you adapt lenses from other systems, like the Zeiss V-mount lenses which go to f/2. So the boost you gain in noise is somewhat countered by not having f/1.4 primes as you would on small format.
The 645Z also requires generally higher shutter speeds for consistent results, with the 55mm lens on I typically shoot at 1/125th, whereas on my old Canon 1/60th would've sufficed.
Although I suppose what ultimately matters are results, and the fact is that I've never felt limited shooting in low light with this camera, for what it's worth. As others have said, the K1 will have maybe a stop worse sensitivity purely due to physics, but it does gain in-body stabilization and you can also use f/1.4 lenses, so in absolute terms it'll be a low-light monster. The 645Z is too, but mainly in comparison to other medium format cameras, and not necessarily all digital cameras.
This of course could change in the future, if Pentax begins optimizing their lenses for the reduced format, a set of 45mm, 70mm and 100mm lenses at f/2 would be absolutely super, although it seems they first want to get some zooms out. About time they updated their lens roadmap, I wonder what's taking so long...