Originally posted by Tjompen1968 The difference between APS-C and FF is small, FF to MF is big.
This statement is not supported by physics.
The difference between formats can be expressed by the so-called "crop factor" which can be calculated as the square root of sensor size quotients. When the aspect ratios between sensor formats are equal then the crop factor describes the quotient between sensor diagonals but since APS-C/FF vs MF have different aspect ratios, the aforementioned calculation approach is preferable.
The crop factor from APS-C to FF is 1.5 (can be slightly higher, depending on the exact sensor dimensions; for Canon it is actually 1.6).
The crop factor from FF to full-MF is 1.64. Hence, the difference between FF and full MF would be rather comparable to the difference between APS-C vs FF as opposed to being "small vs big".
However, the 645Z (and 645D) use a crop-MF format.
The crop factor from FF to crop-MF is just 1.29. Hence, the difference between Pentax APS-C and Pentax FF is bigger than the difference between Pentax FF and Pentax MF.
This means that everything else being the same (here, in particular sensor technology), the jump from APC-C to FF is more noticeable than that from FF to MF in terms of IQ.
Sensor technology has slightly improved from the days of the 645Z to when the K-1 was introduced which is why the 645Z does not even provide the theoretical .37 stop advantage in dynamic range but only a .13 stop advantage (measurements by DxOMark).
So in terms of dynamic range, I'd say it would be pointless to use a 645Z in favour of the K-1, and personally I'd call the
advantages the 645Z has in terms of tonal range and colour sensitivity marginal.
In summary, I wouldn't go for a 645Z if I were after higher IQ but I'd completely understand if someone preferred to work with a bigger viewfinder and preferred the handling of the 645 system overall.