Why exactly do large sensors have more dynamic range

Started 3 months ago | Discussions thread
Erik Kaffehr
Erik Kaffehr Veteran Member • Posts: 5,842
Re: Why exactly do large sensors have more dynamic range
1

Hi,

Part of the explanation may be the pixel size. DR is decided by FWC (Full Well Capacity) / readout noise.

Readout noise depends much on the voltage created over the pixels. That voltage is the charge in the pixels divided by the capacitance of the pixels. Few electron charges over a large pixel yield low voltage which results in high noise.

The Sony A7sIII has huge pixels and very high readout noise at base ISO (around 14.3 EV), the Sony A7rIV has a readout noise around 2.9EV at base ISO.

From Bill Claff's site.

My guess may be that the A7sIII may be limited by 14 bits readout. FWC seems to be around 225 000 e-. 14 bit readout means around 16000 values, so each data number represents about 16 electron charges.

The A7sIII is intended for video, so it needs fast readout.

Note that the sensor switches to high gain conversion at 1600 ISO and readout noise drops to 1.1 e-.

Check the DR values at DxO-mark.

The per pixel values tell a different story.

Looking at the pixel level, SNR (Signal Noise Ratio) is much better on the A7sIII

But, seeing the whole image it is a bit worse off.

What we can see is that the A7sIII doesn't have enough pixels to carry all information from the sensor. But, it is intended for 4K video, not stills.  Another reason 8K video may make sense.

Best regards

Erik

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Erik Kaffehr
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