Hi,
It's in the sensor size.
When I switched from the 20D to the 5D I saw an increase in 3D look from my pictures.
When I shot with a Hasselblad I saw the same thing happen.
That's the reason I switched.
Some people will claim it's only the DOF and you can get the same result with a 1.2 lens or 1.8 on the DSLR but this is simply not true, even on f16 the 3D look from the MF is better.
HOWEVER it is very easily seen on the larger apertures.
Some examples done with the ZD by the way:
Take note how the subject is pushed clear from the surroundings instead of lokking 2D fixed to the surroundings.


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Maybe it's the missing AA filter or microcontrast but I think it's the bigger sensor (larger circle of confussion) because I experienced the same from the 20D to 5D with exactly the same lenses.
Some people don't see it that's true but maybe that's their monitor.
I'm an ISF calibrator and know how the calibration of a monitor (and proper gamma/blacklevel) can affect the look of a picture. On all my setups (including projection) the MF files are just much more 3D.
It's not a placebo I'm afraid.
I saw the effect on the Hasselblad a while ago and have been trying to emulate the effect with my 5D and a 1DsII for almost 5 months and what ever I tried it was impossible, you can get a very nice 3D effect when EVERY element is carefully planned but when I made the switch to MF it was right on the money every shot.