This is the second in a series of posts on the Sony a9. The series starts here.
On the a7x cameras, the quantizing precision of raw files is 13 bits in compressed raw mode with the mechanical or EFC shutter and the drive mode set to single shot. If you use the electronic shutter (called silent shutter in the a7x cameras and electronic shutter in the a9), the precision drops to 12 bits in both uncompressed and compressed operation. If you use continuous shutter mode, the precision also drops to 12 bits in both uncompressed and compressed operation.
What about the a9? Turns out that the camera always drops to 12 bit precision in continuous shutter mode, like its cousins, but stays in 14/13 bit mode with the drive set to single shot.
I made a series of dark field images with the raw format set to uncompressed, and the mechanical shutter set to electronic first curtain shutter (EFCS). When I used the continuous drive mode, I set the rate to low. I turned off all the lens corrections. And, although it shouldn’t matter when making exposures of he back of the body cap, I turned off IBIS, too. I looked at 400×400 crops in the center of the image. This is a worst-case place to look for sensors that are step-exposed in processing, like the a9 must be. It also may be a tough place to look for stacked sensors like the one in the a9.
Here are some histograms. We’ll start with ISO 250, which is high enough to get enough noise to see the shape of the distribution:
You can see that all 14 bits are present.
With the electronic shutter on, things look pretty much the same. Note that there is no material increase in noise level, which was not the casse in the a7x cameras.
In continuous mode:
The two LSBs are AWOL.
Same with the electronic shutter. I’m pretty sure Sony does this to get the frame rate up.
If we move the ISO knob to 640, this is what we see:
14 bits. But look! The noise at 640 is less than the noise at ISO 250. That’s because the a9 uses the Aptina DR-Pix trick of increasing the conversion gain when you go from ISO 500 to 640. Those are the same ISOs as the a7RII.
The histogram is not quite as symmetrical as with EFCS on, but there’s really not much change. Does the blue histogram look a bit strange to you? It does to me, too. Hold that thought until we get to ISO 6400.
In continuous drive mode:
Bits 13 and 14 are gone.
OK, now we’ll move up to ISO 6400:
That is the nicest histogram I’ve seen from a Sony alpha camera. The only thing that keeps it from being perfect is the slight combing in the blue channel. That’s not going to affect your photographs in an of itself, but it does indicate that there may be some raw processing going on.
Looks the same with the electronic shutter.
Turning the continuous shutter drive on costs us two bits.
Turning the electronic shutter on costs us nothing. This is good news.
Next, we’ll take a closer look at the read noise as a function of ISO setting.