Pentax 645D vs Pentax 645Z: read noise comparison

Input-referred read noise at base ISO: Pentax 645D 14.5 e⁻ (ISO 100), Pentax 645Z 3.5 e⁻ (ISO 100).

Pentax 645D

The Pentax 645D was measured at 13 ISO settings from ISO 100 to ISO 1600. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 6.06 DN, or 14.5 electrons at the sensor. From ISO 125 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Full well at base ISO is about 39,235 electrons; unity gain is around ISO 240.

Pentax 645Z

The Pentax 645Z was measured at 34 ISO settings from ISO 100 to ISO 204800. Values are Bill Claff's (Photons to Photos) direct measurements from raw files.

At base ISO (100) read noise is 1.19 DN, or 3.5 electrons at the sensor. From ISO 640 up the camera is effectively ISO-invariant: raising ISO further doesn't meaningfully lower shadow noise. Settings above ISO 2500 don't change analog gain (digital push). Full well at base ISO is about 48,430 electrons; unity gain is around ISO 290.

Pentax 645D vs Pentax 645Z read noise table

ISO	Pentax 645D	Pentax 645Z
100	6.063	1.189
125	6.869	1.474
160	8.112	1.670
200	10.483	2.099
250	13.269	2.585
320	15.562	3.010
400	20.252	3.758
500	26.173	4.691
640	31.125	5.426
800	40.504	6.821
1000	52.346	8.574
1250	62.250	9.987
1600	80.449	12.553
2000	–	16.336
2500	–	19.427
3200	–	19.973
4000	–	25.992
5000	–	31.125
6400	–	38.586
8000	–	50.213
10000	–	59.714
12800	–	59.714
16000	–	77.708
20000	–	92.411
25600	–	119.428
32000	–	154.343
40000	–	184.823
51200	–	213.783
64000	–	276.282
80000	–	328.557
102400	–	424.612
128000	–	548.748
160000	–	652.575
204800	–	837.532

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