What Is Electronic Front-Curtain Shutter

[makoto honda]

The Z7 supports an electronic front-curtain shutter mode. This means the opening of the shutter is done by the sensor without using a mechanical curtain. The Z7's sensor readout time is 1/15 sec (as I learned from this forum), and the focal-plane curtain traversal takes only 1/250 sec (the flash sync time is 1/200 just to be on the safe side). This means the front curtain opens at the scan speed of 1/15 sec, while the rear curtain closes at the scan speed of 1/250 sec. How do you reconcile this big difference in the scan time between opening and closing --- Doesn't this create a huge unevenness in the exposure between the upper and lower portion of the picture?

To me, the electronic front-curtain shutter is a half-baked solution at best - why not use a full silent shutter?

 

[acnomad]

To me, the electronic front-curtain shutter is a half-baked solution at best - why not use a full silent shutter?

I don’t know the answer to the first part of your question, but I believe the “fully baked” solution is known as a global shutter, which, as I understand, is easier said than done!

 

[makoto honda]

I did some reading and thinking, and I now have a better understanding of the question I posed.

First, I take back what I said about the electronic front-curtain shutter being a half-baked solution. In fact, it is a very good solution currently available. So much so indeed there are cameras that make the electronic front-curtain shutter a default mode, or even permanent mode by removing the mechanical front curtain altogether.

My previous question was based on my lack of understanding about the opening and closing speed of the electronics shutter. My terminology is not accurate here, but I am talking about electronically opening and closing the shutter which is not instantaneous. Just like the focal-plane shutter has the front-curtain and rear-curtain scan speed, conceptually, so does the electronics (silent) shutter.

If I continue this conceptual terminology, the electronic front-curtain scan speed can be fairly easily controlled (and can be very fast). The bottle neck is the "rear" curtain scan speed that produces a "rolling" distortion. Because the closing of the shutter entails "readout" of the sensor that can only occur top-to-bottom in row-wise fashion in the current CMOS sensor technology.

So, replacing the slow closing of electronic shutter by the fast, mechanical rear-curtain is a pretty good solution. As I mentioned, the opening speed of the electronic "front curtain" can be easily controlled to match the mechanical rear-curtain's scanning speed, thus averting any exposure unevenness. This is the basic idea behind the electronic front-curtain shutter.

There is some slight uneven exposure issue, however. That has to do with the fact that the scanning plane is slightly different between the mechanical shutter and the electronics shutter. The latter is basically on the sensor surface while the former is a tiny bit off. This creates a slightly different slit width (in high speed shutter) between the upper and lower portion of the frame. The difference is more pronounced if the last lens element is closer to the sensor.

edited: Sunday at 4:20 PM

 

[makoto honda]

The Electronic Front-Curtain Shutter (EFCS) does have a rolling distortion, like a silent shutter, but the effect is NO WORSE than that caused by the regular focal-plane shutter. And yet, the mechanical wear-and-tear is half of the mechanical shutter, since only the upper curtain (rear curtain) moves during the shutter click. So why not use it? For this reason, Nikon introduced the AUTO mode in the SHUTTER TYPE for the firmware version 2.00 for Z cameras. In addition to the choices of MECHANICAL or EFCS, there is AUTO mode, which automatically chooses EFCS based on the shutter speed.