There was a recent post on DPR that showed dramatic effects on bokeh of EFCS. It was a apples/oranges comparison, with the mechanical and EFCS shutter speed different from each other, and the EFCS set to 1/8000, which is clearly out of the range where EFCS usually works well.
Still, it got me thinking, and I ran a quick test with a Sony a7RII and the Sony 90mm f/2.8 macro lens. I set the lens wide open, and the ISO to 640. I lit the subject with a Westcott LED panel, with color temperature set to 5000K and output on full. That gave me 1/1000 second shutter speed. I made exposures with EFCS on and off. Then I turned the light down a stop, and did the same at 1/500 second. I continued along those lines until I got to 1/60 second.
I developed the images in the current version of Lr CC, with default settings except the WB was set to 5000K/0, and a slight exposure boost.
Here are the images:
There are some exposure variations at higher speeds. We expect this. The bokeh differences appear to be minimal to me. However, in theory, there should be bokeh effects of EFCS at high speeds, since the electronic first curtain is in a different plane than the mechanical first and second curtains. Thus, depending on the characteristics of the lens the subject distance, the f-stop, and the place where you look in the image, we should see some differences. They don’t appear to be important here — the exposure variation is more noticeable — but they probably would be at higher shutter speeds.
That’s moot, because we probably wouldn’t use those speeds due to exposure variation. I’ll make some more tests with other lenses.