What is rear curtain flash sync?
Rear curtain flash sync. You might also hear it referred to as rear curtain sync, second curtain sync, or rear curtain flash. What is it exactly? It’s a flash mode that you can put to creative effect, especially when working with slower shutter speeds. Let’s work through it, from curtain to flash, to see what it is and how you can use it.
Table of contents
What are shutter curtains?
Let’s start with shutter curtains. In a DSLR camera, the sensor is covered by two curtains, front and rear or first and second. They’re called curtains because exposures on old plate cameras were managed by actual fabric curtains; the term has stuck. When you depress the shutter button, the mirror flips up, followed by the front curtain to expose the sensor to light. When your exposure duration is complete–as determined by your shutter speed–the rear curtain closes and covers the sensor. The mirror then flips down, and the curtains return to their original positions.
While mirrorless cameras don’t have mirrors to flip up and down, they have similar curtain mechanisms to control when light reaches the sensor.
It’s the curtains that manage the exposure duration.
What is flash sync?
The duration of a flash is so short, 1/500 second or 1/1000 second, that you can time it to coincide with the beginning or the end of your exposure. Flash sync describes just that. It’s whether your flash fires at the beginning or towards the end of the exposure. Front curtain or first curtain sync means the flash fires at the beginning of the exposure. Rear or second curtain sync has the flash fire toward the end of the exposure.
Front curtain sync vs. rear curtain sync
Why do I need to control when my flash fires, you might well ask, and what’s the difference between them?
Front curtain sync is the default flash mode. In fact, some cameras will automatically deploy front curtain sync, even if you’ve selected rear curtain sync, when your exposure time is faster than a certain shutter speed.
You’re only going to select between front or rear curtain sync mode when you’re using a relatively slow shutter speed. If you’re using a fast shutter speed, the short duration of the exposure means that it won’t matter if the flash fires at the beginning or end of the exposure. Hence the default to first curtain sync.
When you combine flash photography with a slow shutter speed, the flash will freeze the motion of a moving subject in the frame while the ambient light illuminates the background.
If the flash fires at the beginning of the exposure it will freeze your subject early on in its trajectory. The slow shutter speed will then record any blur trails. Instead of the motion blur looking as if it is trailing after your main subject, it comes before it. It looks unnatural, almost as if time is travelling in the wrong direction. Switch to a rear curtain sync and this changes. The motion blur will follow the subject, frozen by the flash that fires later on in the exposure.
When should I use rear curtain flash sync?
Use rear curtain flash in slow sync flash situations when you want to freeze your subject while it is motion and ensure that any light trails or motion blur is following it, not ahead of it.
How do I use rear curtain flash?
You might have to look for rear curtain flash sync settings in your camera menu, or select it from your flash itself. It depends on your manufacturers. If you only have a pop-up flash, and no speedlights or strobes, rear curtain sync is often still available to you. Check your manual.
Make sure that your camera is in shutter priority or manual mode, as you want to be able to control your shutter speed. Ideally, you want to set your ISO and aperture to capture the ambient light, which makes manual mode best.
When your flash is in TTL mode, it fires a preflash so that it can meter the scene. The slow shutter speed that you use with rear curtain sync means that the preflash and actual flash can fire far enough apart you notice two distinct flashes. Use manual flash.
You will need to experiment a bit with the flash power, depending on how close your subject is and how strong you want the freezing effect to be.
Finally, if you shoot a brighter subject against a darker background, the contrast between them helps the blur to show up in the image.
Rear curtain sync makes sure that your light trails actually trail your subject, rather than go ahead of them, if you are shooting with flash and a slow shutter speed. It might take a bit of practise and trial and error to get the settings right, but it’s worth it to ensure you capture natural-looking motion.
Rear curtain sync flash fires the flash toward the end of the exposure, rather than the beginning, during a longer exposure.
If the flash fires at the beginning of a longer exposure, it freezes a moving subject first and then captures any motion blur with the ambient light. This looks a bit ‘backwards’ because the blur is ahead of, not behind, the subject.
It is sometimes called second curtain sync, because the flash is synchronized to fire closer to the second curtain covering the sensor, rather than the first curtain uncovering it.
Daniela Bowker is a writer and editor based in the UK. Since 2010 she has focused on the photography sector. In this time, she has written three books and contributed to many more, served as the editor for two websites, written thousands of articles for numerous publications, both in print and online, and runs the Photocritic Photography School.