I own a 645z and have owned and shot everything from M43 up to 8x10 over the years so I do recognise and enjoy the benefits of larger formats, but think we have to be careful not to ascribe qualities that are beyond the realm of what physics permits (as well as what can actually be seen in real life). I was responding to the notion that MF at f5.6 can achieve shallow DOF that has a quality we cannot see on smaller formats (with the suggestion that anyone having actually shot MF would recognise this). Unfortunately it isn't true and isn't close to being true. 80mm at f5.6 on a true 645 format will deliver vastly
more depth of field than f1.4 on FF using a 50mm lens, for example (at a given distance). The 645 and 80mm combo cannot overcome the reduced depth of field possible by the much wider aperture of the 50mm lens, which gives the same angle of view on FF. Sure, shoot the 80mm at f2.8 and the 50mm at f2.8 and the larger format will show less DOF.... but if you seek shallow DOF, clearly you would open the 50mm lens up (whereas many 80mm MF lenses are/were f2.8 wide open).
So, while I agree about the fact that 33x44 has the
potential to offer shallower depth of field than FF,
in practice this is often not the case due to the speed of lenses available (and in the case of the Pentax, options are somewhat limited). Apart from a few specific examples, there is a greater availability of lenses for FF that will offer shallower DOF across a range of focal lengths, especially in the wider focal lengths. A great example would be 90mm on the 645z. Your only native option is the 90mm f2.8 DFA Macro, whereas with full frame, you are looking at 75mm for the same framing. Here, you may have the option of a Summilux 75mm f1.4 which will offer shallower DOF (as would a 75mm f1.8 CV). If you're not shooting Leica M, you could go a touch longer in real terms and grab the Canon 85mm f1.2L (or Nikkor 85mm f1.4), which will offer far shallower DOF and this is simply because big lenses on FF become even bigger on larger formats as the focal length goes up to maintain the same angle of view, meaning the physical aperture is larger etc. In order to produce less depth of field than the Canon 85mm f1.2, Pentax would have to offer a 105mm lens (for same angle of view) significantly faster than f1.8 (I am not sure of the precise maths). But they don't (thought there is a Hassy 100mm f2 and similar). FF usually wins the extreme DOF race (just as they do when you try to figure out what would be needed from MF to produce less DOF than a FF 600mm f4 lens...), but there are other qualities that are strong selling points for the larger formats.
With 'true' 645, achieving shallower DOF than FF is still difficult. 150mm Pentax f2.8 vs (there is no precise angle of view equivalent) 85mm f1.2 L II or 135mm f2 L. The Pentax still does not win the DOF race. It gets much more interesting when you put a 300mm f4.5 on 10x8!
Tonal transition, colour, resolution, lens quality and bokeh are all different issues and even 33x44 (a smaller leap from FF than FF is from APS-C) does offer very real tangible benefits, but depth of field is unrelated to these.
Originally posted by reh321 I guess this depends entirely on your definition of "magical". The Pentax 645Z's diagonal is 26% longer than a full frame's diagonal. According to my rough calculations, that gives a camera with slightly more pixels than a Canon 5DS has but each pixel site is larger than those on a Nikon 810. Thus, by simple physics (not magic) it provides more detail than you can get from any current full frame camera while still more than providing the characteristics that people have always claimed for full frame based on physics. Again using physics, it should have the potential for getting even tighter DOF, depending on aperture.