Hi,
The sensors used in the 50 MP cameras is a different generation from the sensor in the 100 MP cameras.
Reading out a CMOS sensor essentially means connecting a digital voltmeter to the sensor. The digital volt meter is called ADC (Analog to Digital Converter). Both GFX and X1DII use Sony sensors. These have internal columnwise ADCs.
On the 50 MP sensor, there are two ADC-s for each column, one at the bottom and one at the top. Each column on the sensor has around 6200 pixels. So each of the ADCs handles around 3100 pixels.
These ADCs are simple and they are not very fast. Speed is inversely proportional to the precision.
In normal operation the sensor is reset before exposure. Reseting the sensor just essentially means to connect the photodiode to signal ground. This is a very fast operation and it can be done for all pixels simultaneously.
Before exposure the shutter needs to be closed. The sensor is reset, meaning it starts collecting photons. After exposure the shutter is closed, mening that no more photons are reaching the sensor. With the shutter closed the sensor readout starts.
One of the reasons that the 100 MP sensor has faster readout than the 50 MP sensor is that it has not two but four ADC-s on each column. That depends on the BSI (Back Side Illumination). On a front side illuminated device the wiring is in front of the sensor and competes with the photodiode for sensor area. So wiring is kept to minimum.
With the BSI design, the wiring is behind the photodiodes, so it doesn's competer with photodiodes for area.
Therefore, the BSI design can use more ADCs per column.
The time to read out a column defines the blackout time of the sensor. The shutter needs to be closed under readout.
Electronic shutter uses a trick. The sensor is not reset the same time. Each row is reset just before exposure and exposure is terminated by reading the row. But readout is a relative slow process. So reading the sensor may take say 300 ms on the 50 MP sensor. That is essentially the sweep time of the electronic shutter.
The 100 MP BSI sensor works about twice that fast.
I don't know about present times, but developing a new sensor used to take something like three years.
The 50 MP sensor was hanging around for 5 years. Sony later made a 54x41 mm 100 MP sensor, using another pixel design.
With the 100 MP sensor, it seems that Sony has reused the same pixel design for at least four different formats, 54x41 mm at 150MP, 44x33 mm at 100 MP, 24x36 mm at 62 MP and APS-C at 26 MP.
If we look at Sony's own cameras, the A7rII was the first camera to use BSI and it was released 2015. The same sensor was used in the A7rIII. The A7rIV is the first of Sony's 24x36 mm cameras to use a new high resolution sensor. Something like four years for a new sensor generation.
Just to say, I am no sensor expert, far from it. Just tried to write down a reasonable explanation how things work. It was intentionally kept on the simple side.
Best regards
Erik
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Erik Kaffehr
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. it does make the things a little bit clearer for me :-D