Flash units have several menu choices and there is one that is particularly important to understand and become familiar with - the rear curtain sync. The rear curtain sync choice appears on flash units that are a part of the camera or on flash units that are attached to the hot shoe on the top of the camera.
As you scroll through the flash menu you will see the normal flash mode, which is indicated by a lightning bolt. Then there is red eye, slow flash and finally Rear. The key difference between the regular flash setting and the Rear Curtain Sync setting is the timing or when the flash fires. With the regular flash setting the flash goes off as soon as the shutter button is pressed and the shutter opens. With the rear curtain sync selection the flash goes off when the shutter closes - at the end of the exposure. This timing difference creates very different images in some situations.
Generally the regular flash setting is used for anything that is moving or might move, such as wildlife or people - times when you want to freeze the action or where you are not worried about illuminating the background beyond the reach of the flash--usually 6-12 feet.
Rear curtain sync should be used when you are shooting a scene where you want to gather the ambient light and incorporate the flash to illuminate the foreground. Use rear curtain sync for dark interior shots where the exposure is slower due to a high aperture (f/14-f/16) and a low ISO setting.
When you use rear curtain sync the shutter opens and while it remains open the images sensor collects ambient light and then, as the shutter closes, the flash fires and illuminates the foreground.
Example 1: Carmel Mission, California...
The top image above was taken using the regular flash and the bottom using rear curtain sync. In the second image the exposure time was 1.5 seconds, which means that during the second and a half that the shutter was open, the image sensor was processing the image without a flash. Then, as the shutter closed, the flash fired and illuminated the foreground. The important difference is how the information behind the doorway comes to life in the second image, whereas it is just dark in the first image. This is the benefit of using the Rear Curtain Sync setting on the flash unit.
I see that the blue behind the santos is more saturated in the first image, but that is because the sun came out and was shining through a window to the left. You can see the sunlight creating shadows and washing out some of the color.
Now look at two more images that were taken in the same manner:
Example 2: Copper Font at Carmel Mission...
In these photographs (above) of the copper font, the one at top was taken using a regular flash and the one on the bottom was taken using rear curtain sync. Shooting in regular flash mode uses a much faster shutter speed - in this case, 1/15 second. Shooting in rear curtain sync slows the shutter speed and allows the ambient light to be collected, which gives the background detail. The image sensor is collecting all the background information and then the flash goes off at the end of the exposure, lighting up the foreground.
When using rear curtain sync it is a good idea to shoot in the aperture priority mode at an aperture of f/11-f/16 to ensure that the information in the background is also in focus. Shooting at a narrow aperture will also create a slower shutter speed, allowing more time for the background to register on the image sensor.
Rear curtain sync works well in static images where there is background information that is important to the image.
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