Slow Sync Flash

In the article about High Speed Sync, we discussed about focal plane shutters travelling at very high speed so that at any given point of the exposure time only a fraction of the sensor is exposed to light. Now we are going to discuss just the opposite, in slow sync flash mode, we are talking about shutter speeds well below the ones we usually use like 1/15th, 1/8th of a second etc. Now, one may ask since we are using flash to illuminate our subject why do we need such slow shutter speeds. The answer is, to expose the background. In a slow sync flash situation, the aperture controls the flash output and shutter speed controls the ambient light. So your subject will be illuminated by the flash and then the extended shutter speed takes in details from the background.

Photo By Vox

Using slow shutter speeds in combination with the flash allows us many creative opportunities, like first curtain sync, second curtain sync or rear curtain sync. Let us take a look at what these terms mean.

First Curtain Sync

First curtain sync is the default sync mode of all digital cameras. In order to understand first curtain sync we should first have an understanding of how the focal plane shutter of the camera works. Putting it step by step – first curtain sync

•     Shutter button is pressed
•     Front curtain completes travel exposing the entire sensor to light.
•     Immediately the flash fires illuminating the subject.
•     Light from the flash is long gone but shutter still remains open collecting what little light is available to properly expose the background.
•     At the end of the exposure rear curtain moves closing the shutter.

Rear Curtain Sync – Step by Step

•     Shutter button is pressed
•     Front curtain completes travel exposing the entire sensor to light.
•     The extended shutter speed helps collect light exposing the background.
•     At the end of the exposure just before the rear curtain starts to move the flash fires, illuminating the subject.
•     Rear curtain completes travel closing the shutter.

Photo By 51570853@N03

So putting it lightly both appears the same shutter is open for long durations and in between the flash fires. Well it is true and for stationary subjects or scenes it practically does not make any difference if you use front curtain sync or rear curtain sync. In pictures taken with slow sync flash mode ambient light plays a bigger role in the exposure. Switching to slow-sync flash basically affects your picture in two ways; first, a background that would otherwise appear completely dark becomes visible. Second, you can get a good exposure with much less flash power than otherwise required, which usually translates to better lighting with less contrast. It is when you are trying to capture moving subjects that all the magic really happens.

Photo By  Philip campbell

If you photograph a moving object with camera set to front curtain sync, what happens is that the object is exposed by flash at the start of the exposure, the camera continues to record movement and as a result the motion streaks appear to precede the object, creating an illusion of subject travelling backwards. But when shot in rear curtain sync the blurring trails appears behind the subjects much like what you visualize in the real world making it look like the object is moving in high speed.

Photo By Jack Family

Try experimenting with different subjects with both front and rear curtain sync it actually much fun and results turn out much better than you would have imagined, it is surprising how a little suggestion of movement could enliven an image.

In the next article we will discuss about Photography - Lighting - Flash - Understanding TTL Flash – Flash Terminology, Anatomy, Functions and Features

Related Reading

1. High Speed Flash Synchronization
2. How to Trigger your Off Camera Flash
3. Slow Sync Flash
4. Focal Plane Shutter
5. Types Of Flash Synchronization