What is Second Curtain / Rear Curtain Sync
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Example of Rear Curtain Flash Sync
For those of us who understand and leverage the power of having a sun in our pocket, flash is both magical and a necessity. Being able to manage and control the light is integral to success in many cases.
Back in the olden days, we used flash bulbs which had a fairly long flash duration so we tended to use slower shutter speeds to ensure that we got all the light from the bulb. When electronic flash became the norm, we were afforded the opportunity for somewhat higher shutter speeds with flash.
When shutter curtains went to vertical movement from horizontal movement, we got another opportunity for a higher shutter speed because the shutter curtain had less distance to travel.
Electronic Flash Sync Default Operation
This works because electronic flash duration is always much shorter than our shutter speed in normal situations. Today’s cameras offer flash synchronization speeds as short as 1/300 of a second, your model may vary. The longest flash duration averages about 1/1900 of a second, so the reason for the flash sync speed is the shortest shutter speed that allows the sensor to be fully exposed, the first curtain being fully open, and the second curtain not having started to close.
There are times however where we want the combination of a longer shutter speed to document motion blur and still want to freeze our subject. This combination of flash and slow shutter speed is a hallmark of creative flash photography.
The challenge herein is that by default, camera makers set the flash to fire when the first curtain is fully open. This is called first curtain or front curtain sync. It works a charm, most of the time.
When Does First Curtain Let You Down
I’m going to use the example of photographing a moving car with its lights on, at or after sunset. In the normal configuration, the flash fires and captures a clear image of the car, frozen in time, but the shutter stays open and the car moves while the shutter waits to close.
What we get is a sharp car with light trails appearing to come out of the car’s front end, creating the impressio that the car is moving backwards. Generally, we want the light trails appearing behind the car.
Here’s another scenario. I am shooting ballet and have permission to use flash. I know that the dancer is going to leap and I want the motion blur of her leap and to freeze her at the apogee of her leap. The timing of this is really up to my skill and practice but if I do my job, I will get a beautiful motion blur of the dancer going up with the flash going off just as the shutter closes freezing her sharp and well lit while she is in the air. If I am using the normal flash sync, I will get her sharp in the air, but the motion blur will be as she descends.
I enjoy this photography very much, but I never have had the opportunity to photograph a truly great dancer. Marvellous American dancer Gregory Hinds often said of Mikael Baryshnikov that when he went up, he came down whenever he darn well pleased to. The rest of use are more impacted by gravity
Introducing Second Curtain / Rear Curtain Sync
Some makers, in some of their camera bodies allow you to choose when the flash fires whether at the shutter open or just as the shutter closes. At shorter shutter speeds, this distinction does not really matter, but when we want a longer shutter speed, perhaps 1/60 of a second or longer, the ability to choose when the flash fires becomes a boon to the creative.
I am fortunate that all the cameras that I shoot presently have the option to choose rear curtain sync, also known as second curtain sync. I make this a permanent setting, although a camera reset or long term power loss can cause this setting to reset. This means that if I am using shorter shutter speeds, everything works as in the default, but if I am doing a 1 second exposure with flash, the flash only fires at the end, giving me the freeze at the end of the motion, not at the beginning.
This is not to say that freeze then motion is bad, I just find that I prefer the latter most all the time. Check your camera to see if it supports rear/second curtain sync and try it out on a moving subject to see if this appeals to you. Your camera may not support this function and it matters not what flash you use or what flash mode you use. This is an in camera function only. While I personally believe that second curtain sync should be the DEFAULT setting, no manufacturer is asking my opinion. I doubt that they are asking Joe McNally either, and he has professed the same desire and is far more skilled and far better known than I.
Conclusion
Having more choice in when your flash goes off in a longer exposure brings you more creative opportunity. It’s great fun and can open new windows for you.
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I'm Ross Chevalier, thanks for reading, watching and listening and until next time, peace.
