The a7S has a silent shutter mode. When running silent, the mechanical shutter is on vacation, lying back and catching Z’s while the sensor does all the work. The mechanical shutter is a rabbit, completing an exposure in 1/250 second (for shutter speeds faster than that, a slit moves across the sensor, but it takes the slit a little under those 4 milliseconds to make its journey. The fully electronic shutter is a tortoise, taking longer to do its job.
How much longer? I thought I’d find out.
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 2 msec/division, and the amplitude of the generator to just short of maximum excursion for the scope’s vertical sensitivity. I mounted a 105mm f/2.8 Micro-Nikkor on the a7S with a Metabones adapter that I’d modified by removing the foot. I but the RRS plate on the camera and clipped it into a RRS tripod. I set the camera for electronic first curtain shutter, with silent shutter off. I set the camera to manual exposure, and the shutter speed to 1/1000 second – that’s one msec. I stopped down a bit and cranked up the ISO to 16000 to get a good exposure.
I took a picture:
Because of the persistence of the scope’s phosphors, there is quite a bit of residual glow. Try to ignore that and only consider the bright part of the trace. If you do that, you can see that it’s about half a division wide. One division is 2 msec, and we set the shutter to 1 msec, and half of two is one. Isn’t it nice how that worked out? Now notice that the bright part of the image is not vertically oriented, as it would be if every part of the image was exposed in the same 1/1000 second. It’s diagonal. Since scope traces from left to right, the fact that the top of the diagonal is left of the bottom means that the shutter exposed the top of the image before it exposed the bottom. Since the image is upside down on the sensor, that means that the a7S’s mechanical shutter must move from the bottom of the camera to the top.
We can also look at the picture and tell how long the shutter took to go from the bottom of the camera to the top. If we run a line parallel to the diagonal and extend it to the top and bottom of the image like so:
We can see that it takes the shutter about two divisions to go from the bottom of the camera to the top. Two divisions is 4 msec, which is what the synch speed of the a7S is. That’s nice.
Now, let’s leave everything the same except that the camera is in silent shutter mode, and take another picture. This is what we get:
Gee, the silent shutter takes longer to get the job done. How much longer?
Let’s draw our line again:
Oops, it’s off the screen on the right and the left. We’ll just have to guess how many divisions it would take to go from the top to the bottom. I’m guessing two more on the right, and 2.5 more on the left. The line itself is about 2.5 divisions greater in horizontal extent than the ten divisions on the screen. So, 10 divisions on the screen, plus 2.5 plus 4.5, is about 17 divisions. That’s 34 milliseconds. So the Sony a7S silent shutter takes about 34 milliseconds, or roughly 1/30 of a second, to make a complete exposure.