This is the first post in a series on the Sony alpha 7 Mark III, aka the a7RIII. I’ve received some complaints that it’s hard to navigate in this blog, what with its couple of thousand posts and many multi-part series. I’m trying something new. I’ve created a Category called a7RIII. You should be able to find all the posts about that camera in that category. I’ve also added a widget called Category List to the right sidebar, below the Articles widget. There’s a drop-down menu there that you should be able to use to get to all the posts in this series.
The a7RIII has a silent shutter mode. When running silent, the mechanical shutter is open and the sensor does all the work. The mechanical shutter is a rabbit, completing an exposure in faster than 1/250 second (for shutter speeds much faster than that, a slit moves across the sensor, and we’ll see in this post how long it takes to make its journey). The fully electronic shutter takes longer to do its job.
How much longer? Stay tuned.
I set up an analog oscilloscope with an input from a function generator. If you’re going to try this at home, don’t try it with a digital scope; it won’t work. I set the time base to 1 msec/division, and the amplitude of the generator to just over the maximum excursion for the scope’s vertical sensitivity. I mounted a Sony 90 mm f/2.8 lens on the camera. I put a generic RRS plate on the camera and clipped it into an Arca Swiss C1 head on a Gitzo tripod. I set the camera for the mechanical shutter, with silent shutter off. I set EFCS to on and set the file format to uncompressed raw, with the camera on manual exposure, the shutter mode single shot, and the shutter speed at 1/1000 second – that’s one msec.
I used RawDigger to demosaic the images, since ACR, Adobe DNG Converter, and Lightroom don’t yet support the a7RIII.
The mechanical shutter on the a7RIII, like the shutters on all the a7 series, travels from the top of the image to the bottom. Looking at that another way, it travels from the bottom of the camera to the top. The speed is not constant; if it were the diagonal green band would have straight edges. The shutter accelerates as it moves across the sensor. Both curtains accelerate at about the same rate, so that the exposure — the horizontal width of the green stripe — is about the same from the bottom to the top of the image. The width of the stripe is one division, or one millisecond. You will recall that I set the shutter speed for 1/1000 second. 1/1000 second is one millisecond.
If we project where the trailing edge of the green band intersects the top and the bottom of the frame onto the top and bottom of the oscilloscope graticule, and the horizontal distance in divisions between those dots will give us the shutter travel time. I put it at 3.4 divisions or the same number of milliseconds. That’s about 1/250 of a second, which is about average for mechanical shutters these days.
Now, let’s leave the scope set up the same way, switch the camera to silent shutter, and make another picture:
The electronic shutter travels in the same direction as the mechanical one, from the top of the image to the bottom, or the bottom of the camera to the top. You can see that the electronic shutter is much slower. To find out how much slower, we’ll have to change the scope’s time base. I set it to 10 milliseconds per division, and made another exposure:
I put the scan time at 7 divisions or 70 milliseconds. That’s about 1/15 of a second, or about the same speed as the a7RII silent shutter.
But if we set the file format to compressed raw, and the shutter mode to continuous (C), things move a bit faster:
That’s almost twice as fast, or about 1/30 second.