The physical truth behind bokeh clipping and uneven exposure under extremely high shutter speeds (1/2000s+) and large apertures with camera electronic front-curtain shutter (EFCS)

1. Working mechanism and physical architecture of the three shutter modes

Before understanding this physical phenomenon, it is necessary to clarify the three shutter working modes of modern mirrorless cameras:

1. Purely mechanical shutter (Mechanical Shutter):

• Exposure starts: the first mechanical curtain (front curtain) runs downward and opens, exposing the sensor;
• End of exposure: The second mechanical curtain (rear curtain) runs downward to catch up and close, blocking the sensor.
• Both front and rear curtains are completed by real mechanical blades located on the same physical plane in front of the sensor (approximately 2~3mm from the CMOS surface).

2. Full electronic silent shutter (Electronic Shutter):

• There is no physical curtain movement, and it relies solely on the sensor to turn on the light-sensing line by line (electronic front curtain) and stop reading line by line (electronic rear curtain). Both curtains are done on the CMOS chip physical silicon surface (0 distance).

3. Electronic Front-Curtain Shutter (EFCS - Electronic Front-Curtain Shutter):

• Exposure starts: relying on the sensor row reset signal electrical pulse to instantaneously activate the photosensitizer (electronic front curtain, located on the CMOS silicon surface, distance = 0mm);
• End of exposure: The upper physical mechanical blade sweeps down to cut off the light (mechanical rear curtain, located in front of CMOS, distance = 2~3mm).

2. The spatial physical cause of Bokeh Truncation: asymmetric slit parallax

Why do purely mechanical shutters and purely electronic shutters not cut corners, but only EFCS does?

• Slit Scanning at high speed shutter:
• When the shutter speed exceeds the flash sync speed (such as 1/2000s ~ 1/8000s), the rear curtain has already begun to catch up and close before the front curtain is fully opened;
• The sensor does not expose the entire frame at the same time, but an extremely narrow micron-level moving gap formed between the front and rear curtains sweeps across the entire frame.
• Geometric shadow casting in three-dimensional space:
• Large aperture lenses (such as 50mm f/1.2, 85mm f/1.4) have a huge exit pupil, and the angle of the converging light cone is extremely wide (light enters the corner pixels at a large tilt angle);
• Front curtain (electronic) is at ground level (0mm) while Rear curtain (mechanical) is suspended 2.5mm in the air;
• When the large-angle edge light incident obliquely passes through this spatially misaligned slit at high speed, part of the light is intercepted and blocked in advance by the mechanical rear curtain at the height, while the light from the other side enters unimpeded;
• Geometric truncation effect: On the out-of-focus circle of confusion (Bokeh Ball), this one-way spatial occlusion is directly manifested as the lower or upper half of the light spot being sharply cut off, and the originally round and beautiful light spot becomes an ugly and deformed semi-flattened angle crescent spot!

3. Uneven exposure at the top and bottom of the screen at high speed and stripe faults (Uneven Exposure)

In addition to background blur collapse, EFCS can also induce fatal light and dark shadow lesions at 1/4000s ~ 1/8000s:

• Physical drift of effective light time:
• The mechanical rear curtain is a set of metal blades with real mass, and its acceleration curve is affected by gravity, spring tension and air resistance;
• The electronic front curtain is an electrical signal at the speed of light, and the scanning speed is absolutely constant;
• In an extremely narrow slit (1/8000s, slit width is less than 1mm), the difference in the movement speed of the mechanical rear curtain at the top and bottom of the picture cannot be matched 100% synchronously with the uniform speed of the electronic front curtain;
• Scale gradient light and dark color cast: When shooting a pure blue sky or a pure white wall, a 0.3EV ~ 0.5EV light and dark step fault will appear at the top and bottom of the screen, and even zebra stripes will be stimulated under some high-frequency artificial light sources.

4. EFCS uses boundary and shutter modes to switch golden decision trees

Use the shutter mode scientifically without having to worry about EFCS:

1. Shutter speed (\le 1/1000s): Feel free to turn on EFCS:

• At this time, the slit width is extremely sufficient, the shadow truncation caused by spatial misalignment is minimal, and the light spot deformation is completely undetectable to the naked eye;
• And at this time, EFCS can completely eliminate the "Shutter Shock" caused by the mechanical front curtain opening, greatly improving high-pixel sharpness at 1/30s ~ 1/250s, while minimizing shutter lag (Shutter Lag).

2. When the shutter speed is (\ge 1/2000s) and paired with a large aperture ((f/1.2 \sim f/2.0)): resolutely turn off EFCS and switch back to the full mechanical shutter! ：

• When shooting large-aperture portraits in bright outdoors, you must enter the menu to switch the shutter to Mechanical Shutter;
• The mechanical front curtain and the mechanical rear curtain fly in the exact same spatial plane, perfectly ensuring that the out-of-focus circular spot is full and round, without any corner deformation, and the full-frame exposure is absolutely uniform!

3. Curtainless relief for flagship models (Z8/Z9, A9III): The stacked sensor and global shutter completely eliminate the mechanical shutter component, sending this problem to the history museum at its root.

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