What is Slow Sync Flash?
Slow sync flash is when you fire your flash either at the beginning or end of an exposure that’s slower than normal, for example 1/8 of a second. Anyone with experience behind a camera knows that it’s very hard to hold the camera steady enough for a sharp exposure at this sort of speed, and that’s where the flash comes in. By firing the flash, you freeze the motion and collect light trails in the remaining time, creating this rather cool effect, like in the photos below.
Why You Would Use This
The main advantage of having a longer shutter speed is that it allows you to collect more ambient light into a photo, and the flash firing helps to maintain a relatively sharp image. When you’re forced to shoot in low light conditions, but still want to maintain plenty of the ambient light detail without a high ISO, your best option is to use a slow sync flash. Below is a photo shot at 1/8 of a second with the flash firing, and below that is the same photo shot at a comfortable handheld speed of 1/50 of a second, again with the flash firing. Notice the different in background detail in the ambient light and how the movement is almost unnoticeable in the first photo with the shower shutter speed.
How To & Movement
All you need to get this to work is a camera with a flash and the ability to change the shutter speed, after that you have 2 options; 1st (front) curtain sync and 2nd (rear) curtain sync. Without going into too much unnecessary detail about this means, 1st curtain flash means that the flash is fired at the beginning of an exposure, and 2nd means that the flash is fired right before the end of the exposure. The importance of choosing the right one comes down to the fact that there is invariably going to be movement when you take a photo, and when you fire the flash dictates the type of blur you’re going to get.
1st Curtain Sync.
This is the standard way of shooting with a slow sync speed and doesn’t require any special setting up to achieve, other then adjusting your shutter speed. The flash will fire when you press down the shutter and the camera will continue to record any light for the remainder of the exposure. The main issue with shooting like this is that any movement will record the light over the top of the original exposure, dulling it and making it appear less sharp. Have a look at the photo below and you’ll notice that the light streaks come after the original exposure and over the top of any potential subjects.
2nd Curtain Sync
When you change your flash setting to fire at the end of an exposure, you exposure your subject over the top of any streaks or blurs that have taken place in the time that your shutter has been open. Any light streaks now go towards and end at your subject, helping to draw the attention to them. Most cameras are different, so look in your manual, but to set your flash to rear curtain sync on an external unit, it usually just consists of the click of 1 button. Adding movement to your camera while taking the photo produces some really cool effects like in the photos below. I moved the camera from side to side for the first photos, up and around the sky in the second one and I zoomed in during the exposure of the third one. There’s plenty of fun to be had playing around with this technique, but I recommend you have a good look around the scene first to see what available light you have and to see what would work for your photo.