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Many recent cameras allow using electronic first curtain shutter with mechanical second curtain, avoiding the huge rolling shutter drawbacks of fully electronic shutter and at the same time avoiding the need for mechanical first curtain.

In fact, some cameras (like EOS RP) have electronic first curtain only.

There are some youtube videos that explain how EFCS affects bokeh rendering:

...and many, many more.

But why does the effect occur?

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The reason for the phenomenon is that at wide apertures in good light necessitating a fast shutter speed, most of the exposure is coming from a very narrow traveling slit between the first and second curtain.

The curtains before and after the slit are not at the same height, however.

The electronic curtain travels where the photosites are.

The mechanical curtain travels at significant distance from the sensor. In particular, there is an anti-aliasing filter and perhaps some protective glass on top of the sensor, and only then the shutter curtain.

Because the curtains are at different heights from the sensor, parallax effect becomes significant. The faster the shutter speed, the smaller the traveling slit is, and the problem becomes more pronounced.

To fix the issue, the shutter speed needs to be slowed down. Firstly, if the camera is not already at its lowest ISO setting, the ISO should be lowered to the lowest ISO. Secondly, if the shutter speed is still too fast causing the effect to appear, an ND filter could be used.

However, the best fix is to use shutter curtains that are at the same height from the photosites. So, if the second curtain is mechanical, the first should be mechanical too. Unfortunately, with all cameras, this is not possible.

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  • Wow, thanks for the explanation. I always wanted to know, why this is the case. – Kai Mattern Apr 26 at 17:34
  • I really want to upvote this answer, but I feel it needs to be made clearer when you are talking about a physical shutter curtain and when you are talking about the virtual electronic second curtain as you discuss the different distance from the sensor for each. – Michael C Apr 27 at 14:34

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