as I said many times ,high ISO is not the real reason why you want to a 5d2 or a d700 but their DOF control is the main reason why you want a full frame for serious work.
and most of lenses ,especally old primes work better on FF than crop...........or APS-C.
until you switch to full frame you wont get it.
and cost of a full frame system is not that much more than a k7 based sytem, and pentax has no real fast primes to get even close to the legendary 85f1.2l2 on a full frame.
it is not high ISO but speed and DOF control why full frame is better, and full frame has the best wide angle lenses , the 16-35f4VR , 14-24, 16-35f2.8L2 are much better lenses than any of Pentax wide angle zooms.
I am not saying or even know if op likes pentax sytem but I am talking about the real difference between full frame based system and K7 based system, they are not even comparable, where is f0.7 Pentax prime?
Unless you have a f0.7 lens on your k7 , you wont be able to touch the 85f1.2L on a full frame.
as for the size , well, the k7 is small but not that much lighter than my 5D2 or my d700, my 5d2 weighs 800g and the k7 weighs 675g, is that a huge difference?
the D700 is a bit heavier but with a small Nikon or Voiklander primes , it is actually very small , I mostly use my D700 with a couple of primes (AFS50f1.4G and Voiklander 20f3.5) for all my casual photos....... and it is smaller than Pentax K7 with DA16-50f2.8 lens...............and I know this lens had CA issue and SDM failure issue, I had it before.
dont get me wrong , I had the k7 and liked the body ,ergonomics ,etc but it is simply horrible IQ wise and Pentax has no fast primes , so as much as I wanted to love it I sold it for another D700 , to back my 5D2 up.
any way hope Pentax gets it right but in Japan , most of big sites and papers talking about Hoya is planning to sell Pentax division to JVC or Sharp.
so, it is not safe to go with them , I just hoped OP to make fully informed decsion , that is all.
I have no brand preference , in fact I prefer Pentax to Nikon if they had a full frame with Nikon or Canon sensor since they are my local company and my dad used to work for them as a designer...............but now, they make not so nice cameras in terms of aF, IQ and speed or operation.
I used to be a minolta user so I am very very scared to invest any money on some unknown system or dying sytem and I do not want other people to experience the same horrible losing their mount as I did , I had to switch to Canon because Minolta decided to quit DSLR business in 2006.
Also , you must know that there is no real market for the 645D , every body sane says it is a big mistake, real pro buy Hassy , Phase one , Leaf or Mamia .etc and Pentax said the 645D is for amature landscapers , but I dont think many amature shooters buy it or afford it,or even those who can afford it avoid it , only a very few collector get it and it is even not marketed world wide but only in Japan.
Japan is a tiny country and it is obvious it won't sell well there, the economy is horrible in Japan, most of people here have no money.
So, I think no matter how good Pentax bodies are , Pentax is a brand to avoid , not safe, all know that.
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cheers, and good shooting!