Optical and physical differences between front-curtain sync and rear-curtain sync of digital camera flash in slow shutter dynamic photography

1. The double exposure nature of Slow Sync

When using flash with a slow shutter speed (such as a 1/2 second to 2 second long exposure) at night, the entire film is actually drawn by two completely different physical energies:

1. The first force: Freeze Moment of flash:

• Flash duration is only 1/1000s ~ 1/10000s;
• The intense flash of light accurately ‘imprints’ the subject’s clear outlines and microscopic textures on the negative with an almost absolutely static coagulation force.

2. Second force: Continuous flow of ambient low light (Motion Trail):

• During the exposure period of up to 1 second, the luminous points on the street lights, neon lights and the subject move with the movement, drawing continuous flowing translucent dynamic light trails (Light Trails) on the film.
• The core life gate that determines success or failure: Does this flash bomb that explodes instantly ignite at the very beginning of this 1 second, or at the very last moment of ?

2. Front-Curtain Sync: Counterintuitive ‘reversing illusion’

The factory default flash sync mode of most cameras around the world is "1st Curtain Sync":

• Mechanical Timeline Logic:
• Step 1: The moment you press the shutter, the front curtain opens completely and the flash fires immediately!
• Step 2: The subject is instantly and clearly frozen in the initial position by the flash;
• Step 3: For up to 1 second, the camera continues to open the door for exposure, while the subject continues to run forward, and the reflective objects on the body continue to pull out a light trail in front of the screen;
• Step 4: Close the rear curtain to end the exposure.
• The Reversing Illusion:
• The final photo appears: the clear subject stays at the back of the frame, and the long light track extends straight from the subject's chest or front of the car to the front of the body**!
• In human visual subconsciousness and brain physical cognition, the motion shadow of an object should always stay behind the object;
• The picture presented by this kind of front-curtain synchronization looks like the subject is strangely reversing (reversing) at an extremely high speed, seriously destroying the forward psychological potential of the movement!

3. Rear-Curtain Sync: a natural miracle of compliance mechanics

Turn on "2nd Curtain / Rear Curtain Sync" in the camera or flash menu:

• Subverted Timeline Action:
• Step 1: The moment you press the shutter, the front curtain opens and the flash does not light at all and remains silent!
• Step 2: During the long exposure period of up to 1 second, the subject runs and jumps forward. In the darkness, only the streetlight track outlines a winding motion track light strip on the film;
• Step 3: At the last thousandth of a second when the exposure is about to end and the rear curtain begins to close, the flash explodes with precision**!
• Step 4: The rear curtain sweeps closed instantly.
• Absolute balance of mechanics and aesthetics:
• The final photo shows: the clear, sharp, and slender subject stands proudly at the forefront of the movement trajectory, and the gorgeous and dreamy light tracks drag straight out from behind the subject like flowing water**!
• Perfectly consistent with the momentum cause and effect of speed, sprint and smear in the real physical world, the tension explodes!

4. Practical three-step lens movement for night scene portraits with slow door and rear curtain synchronization

Taking emotional portraits on neon streets at night:

1. Parameter Matrix Benchmark:

• Set the mode to M position;
  -Set the shutter speed to 1/4s ~ 1s (recommended 1/2s);
• Set the aperture to f/4 ~ f/5.6 (keep the background neon light circle);
• ISO is set to 200~400 to ensure moderate background low light exposure.

2. Rear curtain synchronization locking and hand movement skills:

• Confirm that the flash screen displays the [REAR] mark;
• After pressing the shutter, the photographer holds the camera with both hands and swings it smoothly sideways or in an arc around the optical axis of the lens (Camera Shake/Twist), allowing the background street lights to draw gorgeous rotating lines of light on the screen;

3. Final freeze:

• At the moment when the pull was about to end, the rear curtain flash exploded with a "pop" sound, and the model's delicate facial features and hair were frozen extremely sharply in the center of the brilliant light track storm, and the business atmosphere was instantly filled!

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