In the article titled Understanding Flash Sync Speed, I discussed in details how focal plane shutter works and what maximum sync speed is. What was left out was the two different flash sync modes: front-curtain vs. rear-curtain sync.

In front-curtain sync, the flash fires immediately after the first (front) curtain opens completely; in rear-curtain sync, the flash fires just before the second (rear) curtain starts to close.  To understand the differences they make, let’s use a simple two-image model.

The image sensor continuously capture the image formed by the lens on the sensor surface when the shutter is open. There is one image captured by the camera in each exposure but you can imagine the sensor captures two images: one image formed by ambient light and one image formed by the flash. The two images are then superimposed together. In front-curtain sync, the flash image is captured first followed by the image of the ambient light; while in rear-curtain sync, the flash image is captured after the image sensor captures the ambient image.

The flash image is motion-freezing because the typical flash duration is very short: ~1/10000 second to ~/1000 second. But the ambient image may not be depending on the shutter speed and the relative intensity of the ambient light.

When photographing in a completely dark room, there will be no difference no matter which sync mode you choose. In practice, when the light from flash dominates overall exposure, the two flash sync modes do not differ much either. In the two-image model, the final image you get is mostly the flash image.

Things will be different when the ambient light is strong enough and/or the shutter speed is low enough that the ambient image is clearly visible. If the shutter speed is low, the blur caused by camera shake or subject motion can be captured and blended with the flash image.  Sometimes the ambient is weak and the shutter speed is not really slow but the subject is fast-moving and it contains bright spots that can still be captured during the time the shutter is open. One example is the head/tail lights from a moving car.

When the shake or motion is completely random, it doesn’t matter (contrary to what some experts said) if you use front-curtain or rear curtain sync mode. The final image is the simple addition of the flash image and ambient image. There is no way to tell which one is on top or bottom. It is different when the shake or motion is not random. A moving car leaves its light streaks behind with rear-curtain sync mode but an unnatural blur in front of the car with front-curtain sync mode.

From what is said here it sounds like it is better to leave it always in rear-curtain sync mode. This may be true if the camera exposure mode is M (manual) or S (shutter speed priority). On Nikon DSLR cameras, selecting the rear-curtain sync mode when the exposure mode is in P (Programmed Auto) or A (Aperture Priority) causes the camera to go automatically into Slow Sync mode, in which the camera will choose a shutter speed that exposes the ambient to a standard level. In Front-curtain sync mode and P/A exposure mode, the shutter speed will be normally between 1/60s (determined by Flash Shutter Speed custom setting) and maximum sync speed of the camera. In typical low light shooting situations, such shutter speed is typically fast enough to make sure the flash exposure dominates when ISO and aperture settings are properly set. In some cases it may cause blur problems. For example when shooting in-doors with available sunlight the shutter speed may be ~1/30s for a proper exposure without flash. So 1/60s shutter speed with flash can leads to fair amount of blur.

Some tips

  • If you want to freeze all motion, make sure the flash exposure dominates or the shutter speed is high enough. To do that you may have to use manual exposure mode to overcome the tendency of the camera doing things on its own.
  • If you want to properly capture non-random motion, use rear-curtain sync mode.
  • Use tripod with slow shutter speed when ambient light is significant to reduce the blur caused by camera shake. However this will not help with motion blur caused by moving subject.

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