Announced in April 2014 as the replacement to the 40-million-pixel Pentax 645D, the Pentax 645Z is a medium-format DSLR with a Sony sensor that measures 43.8 x 32.8mm. With 51.4 million effective pixels, the CMOS chip produces 8256 x 6192-pixel images that make 27.5 x 20.6-inch (69.9 x 52.4cm) prints at 300 ppi.

There’s no optical low-pass filter over the sensor, but there’s an AA filter simulator that uses a sensor-shifting Shake Reduction system if moiré patterning is an issue.

Unlike some medium-format cameras, the Pentax 645Z’s sensor is integrated into the camera so it can’t be changed or upgraded with a different back. However, the camera has the look and feel of an overgrown 35mm or small-format DSLR, with weatherproofing and dust-proofing that makes it more at home in the field than some other medium-format models.

At this writing, the 645Z’s pixel count is beaten only by such digital backs as the Phase One IQ3 (which is available with 50, 80 or 100Mp) and the Hasselblad H6D-100c.
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https://www.dxomark.com/pentax-645z-sensor-review/