Have you tried out slow sync flash yet? Well you should. It’s a really fun digital photography trick that never fails to produce some interesting results. I like to think of it like extended exposure with a few extras. You can create a neat effect that combines the motion blur from the night lights with the frozen motion you get from using a flash.
The picture above (taken by Flickr User: jdanvers) was taking using 2nd curtain slow sync flash. There is a difference between 1st curtain and 2nd curtain flash, and we’ll get to it in a moment. For now, the most important thing to notice is how clear and focused the Coke can appears compared to the rest of the shot.
How is that possible? Is the photographer using a tripod or something? Nope. Not at all. Here’s what’s really going on...
The flash is going off just before the shutter closes, bouncing light off of the can for a split second. The light then gets recorded on the camera’s sensor, producing a crystal clear image. This should come as no surprise, really. It’s the same thing that happens whenever you take a picture using flash. That’s why we use it to overcome the absence of light found in most indoor shooting situations.
But there is something different going on here. Most of the time, when you shoot with flash, you also shoot using a rather fast shutter speed. After all, if your camera's flash is producing the light, why should the shutter stay open longer to record any extra light? Slow sync mode is different. Your camera uses the flash but keeps the shutter open longer as well. This creates the light trails you see in the photograph above.
1st curtain and 2nd curtain modes
There are two ways to use slow sync flash mode. You can have the flash fire at the beginning of the long exposure, or you can have it fire at the end. Whenever you hear someone talking about 1st curtain and 2nd curtain flash modes, this is what they’re referring to. Depending on the shot, one might look better than another.
Why is this process called 1st and 2nd curtain? Glad you asked. It has to do with the way the shutter mechanism in SLR cameras is built. You probably thought your camera contained a single shutter. In reality, your shutter is composed of two shutters, one that opens at the beginning of the shot and another that opens at the end of the shot.
Every time you take a picture, the first curtain opens to start the exposure, and the second curtain closes to end the exposure. This is how it’s possible to get shutter speeds as fast as 1/1000s and above. It’s really two separate mechanisms firing on their own.
So, when we talk about 1st curtain flash and 2nd curtain flash, we mean the curtain that triggers the flash. If you’re using 1st curtain flash mode, the flash will trigger after the first curtain opens the shutter. In other words, it will trigger at the beginning of the exposure. If you’re using 2nd curtain flash mode, the flash will trigger just before the second curtain closes the shutter. That happens right at the end of the exposure.
Why use one over the other?
It all depends on the effect you’re trying to create. The photo of the Coke can works because the Coke can is in front of the light trails. In order to get that effect, you need to use a 2nd curtain flash. Here’s why.
As the can moves across the frame, it reflects a small amount of light, creating the distinct trail effect you see. Then, at the very end of the shot, the flash is fired and the can is illuminated. The can is in the 'end' of the trail because it was the last thing captured. If the photographer were to shoot the same picture with 1st curtain flash, the can would appear to be at the start of the light trail. It wouldn’t look 'right' to us.
Most of the time, you’ll probably be using 2nd curtain flash because you’ll want your subject to appear in front of the streaks. Use 1st curtain flash when the moment of 'flash' is important - like to highlight a specific mode of a dancer. (Thanks to Chris Schonwalder for pointing out the best use of 1st curtain flash.) If you’re ever unsure, why not experiment with it? Try both modes and see what you like best.
How can you access 1st curtain and 2nd curtain modes?
The ability to control slow sync flash tends to come with higher end cameras and digital SLRs only. Each tends to have its own way of describing these two modes as well. Some camera makers call it “front curtain / rear curtain,” or “front mode / rear mode.”
On a Nikon camera, you can get to the two slow sync flash modes by holding down the lightning bolt icon and twisting the dial on the back of the camera. SLOW mode refers to 1st curtain mode, and REAR refers to 2nd curtain mode.
On a Canon camera, it’s built right into the main menu. Just select the “shutter sync” option, and go from there. Canon calls theirs 1st curtain and 2nd curtain so there’s no ensuing confusion.
With this slight adjustment, your slow sync flash photographs will start looking a lot nicer. Please let me know your results by leaving a comment below.
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