What is Slow Sync Flash?
Slow sync flash involves firing your flash either at the beginning, or at the 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 a camera steady enough for sharp exposure at these speeds, that’s where the flash comes in.
By firing the flash, you freeze the motion, collecting light trails in the remaining time which creates this rather cool effect, as shown in the photos below.
Why You Would Use This
The main advantage of a longer shutter speed is that it allows you to collect more ambient light into a photo with the flash still firing 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, it’s usually best to use a slow sync flash.
Below is a photo shot at 1/8 of a second with the flash firing. Below that is the same photo shot at a comfortable handheld speed of 1/50 of a second, again with the flash fired.
Notice the difference in background detail in the ambient light and how the movement is almost unnoticeable in the first photo with the slower shutter speed.
How To & Movement
To get this to work, all you need is a camera with a flash and the ability to change the shutter speed. After that you have two options: 1st (front) curtain sync or 2nd (rear) curtain sync.
Without too much unnecessary detail, 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. The point at which the flash is fired 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, other than adjusting the shutter speed, it doesn’t require any special set up to achieve.
The flash will fire when you depress the shutter and the camera will continue to record light for the remainder of the exposure.
The main issue with shooting like this is that light due to movement will be recorded over the top of the original exposure, dulling it and making it appear less sharp. Have a look at the photo below. You’ll notice that the light streaks come after the original exposure, over the top of any potential subjects.
2nd Curtain Sync
When you change your flash setting to fire at the end of an exposure, your subject is exposed over the top of any streaks or blurs that took place in the time that your shutter was open.
Any light streaks are now directed towards, and end at, your subject. This helps 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 consists of clicking just one button.Adding movement to your camera while taking the photo produces some really cool effects, demonstrated in the photos below.
I moved the camera from side to side in the first photo, up and around the sky in the second, and zoomed in during the exposure of the third.
There’s plenty of fun to be had playing around with this technique but I recommend having a good look around the scene first to determine your available light and work out what would work in your photo.
A note from Josh, ExpertPhotography's Photographer-In-Chief:
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