An Explanation of 2nd Curtain Sync Flash (or Slow Sync Flash)

An Explanation of 2nd Curtain Sync Flash (or Slow Sync Flash)


Let’s look at the sequence of events when combining flash with long exposures. When the flash fires during exposure, the normal sequence looks something like this (note there may be some slight variation among different camera brands or flash modes):

  1. Press shutter button.
  2. Curtain A opens.
  3. Flash fires.
  4. Frame is open for some period of time (as determined by your shutter speed).
  5. Curtain B closes the frame, ending the exposure.

This is the normal sequence and doesn’t really have a name, except to help differentiate it from our next scenario. With Second Curtain Synch (also sometimes called Slow Synch) the sequence goes like this:

  1. Press shutter button.
  2. Pre-flash fires so the camera can measure and adjust the intensity.
  3. Curtain A opens.
  4. Frame is open for some period of time (as determined by your shutter speed).
  5. Flash fires.
  6. Curtain B closes the frame and ends the exposure.

To understand the significance, imagine you are taking a picture at night in which moving lights will be visible during the exposure. During the exposure, the lights move across your frame and you are not panning.

During the first (normal) exposure, the flash illuminates the subject on one side of your frame. The lights then form streaks across the frame right over top of the subject, resulting in a weird looking image.


In the second scenario, the shutter opens and the lights record their streaks. Then the flash illuminates the subject. The streaks from the lights lead up to the subject, giving the illusion of speed and creating a generally cool effect.


Depending on the speed and direction of movement and the intensity of the lights, it doesn’t always work this way but one of the great things about digital is that you can review your results and try something different if you didn’t get what you were expecting.

It bears noting that you can’t precisely control exactly when the flash fires when using second curtain synch. So it’s possible in our example scenario that the subject could be out of the frame before the flash goes off. Second curtain synch requires some practice, a bit of finesse and sometimes luck to use well.

This post is an excerpt from Jeffrey’s book Photography Basics.

PS: Check out these Amazing Examples of Slow Sync Flash

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Jeffrey Kontur is the author of two books on photography. You can see more of his work at More Satisfying Photos where he also publishes and distributes his own email newsletter with free weekly photography tips.

Some Older Comments

  • Herbert August 15, 2011 02:15 am

    can you use 2nd curtain sync or the slow sync into offset flashes?

  • liz read September 4, 2010 12:01 am

    Great stuff - rear/front sync - Is there some way to play this game when using long exposures - no flash? Or is the registration of movement consistent?

  • Malta McG April 16, 2010 11:24 pm

    Love the comment by Richarquis.... ...I'm also wasting loads of work time on this page! Actually, just got back from the camera store over lunch and was checking out a Sigma EF -530 DG Super flash to couple with my Canon 400D.

    Why am I buying a flash and researching second (or rear) curtain flash technique? Because I accepted a gig as nightclub photographer two days ago.. ...with zero experience in nightclub photography :) I'm a quick learner though, and tutorials like this are very helpful.

    Also, make sure to Google "nightclub photography" for a variety of pages and forum threads dedicated to this technique, with recommendations on f-stop, shutter speed, lenses etc...

    Great article, thanks.

  • CDA April 14, 2010 12:39 am

    Nice article, but I don't think the pictures are the most representative ... you could've found some better examples.

  • sarah louise johnson March 20, 2010 10:04 am

    been wanting to try this for ages properly going to experiment more!!!!!!

  • richarquis February 18, 2010 12:29 am

    I hated using flash too, as it either made the subject too harsh, or, it ended up giving me a near black screen. The first mistake was being too close, the second too far, from my subject. As a consequence of the first, I'd overcompensated with the second. Then I discovered I could adjust my flash exposure (Another reason to read the manual and click on tutorials like these.) Since I discovered I could reduce that harshness, I could get nice and close to my subject, and get a more natural tone. Then I started reading about backing off and increasing the ISO to extend the flash range - something I still need to experiment more with. Now, getting better results, I've really started to enjoy using it. But this effect is by far the best thing I've learned yet. I need to practice timing and movement to get the right effects, and the most interesting shapes with the snail trails, but damn, what a lot of fun it is. This site is now bookmarked in my pc, and I plan on wasting as much work time as possible on it reading tutorials, and my own time implementing what I've learned. Thanks for making a slow day interesting guys. I'll post some examples when I've got a better technique down.

  • Jeff Kontur August 26, 2009 08:52 pm

    Actually the reason (and I guess I should have made this clear above) is that with "ordinary" flash, the camera assumes that the flash is the primary light source. Therefore, it has total control over the light in the picture. With slow and second curtain synch, the camera assumes that the flash is supplemental to some other light source. Therefore, it has to measure the other light source and also measure the flash against it to determine the best output to use.

  • wageslave August 26, 2009 01:54 pm

    Can anyone explain why the Second Curtain Synch needs a pre-flash at the beginning (supposedly to adjust intensity), but not the First Curtain Synch (i.e, the normal sequence)? Isn't the exposure (shutter speed, aperture and ISO) already determined before *any* flash is fired?

  • Jeremy March 3, 2009 06:12 pm

    I remember you posted a guide about this a year or so ago, illustrated with some really beautiful pictures. The samples here really don't interest me that much, but these ones were really cool. Slow sync flash seems to allow you to really experiment and get arty.

  • Lou March 1, 2009 06:42 am

    Don't worry about "calling me out". I want things to be accurate, not "according to Lou". I did associate dragging the shutter with slow sync, not 2nd curtain. I thought slow sync was just using a slower shutter speed, different from actual 2nd curtain sync. I did enjoy this article and all the comments. I use flash quite often. I don't really use the 2nd curtain function much but should experiment more with it. This is the first time I've looked into this web site and I did join and post some in the forums. Thanks for your work here.

  • Eric Rossicci March 1, 2009 06:33 am

    I am new on this School of Photography and am not too sure if this is the correct spot to ask a question.
    I own a Canon 40D and the 580 ll Canon Flash. I noticed that if I use the Auto setting on the camera and try to take flash pictures of say; dancers, the pictures are blurred. Obviously because of the slow shutter synchronization. What would be the correct setting for fast action photography with this flash? I also have the Better beamer to extend the range of the flash.
    Sorry if I posted this in the wrong place, you have to forgive me, I am still using baby steps here ..

  • Jeffrey Kontur February 27, 2009 11:17 pm

    Because I don't shoot Nikon, I can only venture an educated guess. I think the article covers regular flash and rear flash. I believe that slow flash is what Lou mentioned above. It is also called "dragging the shutter". (Sorry to call you out Lou but dragging the shutter is actually something slightly different from second curtain synch.)

    With dragging the shutter, the camera combines a long shutter speed (to capture ambient light) with flash. Naturally the next question is whether the flash fires at the beginning or end of the exposure. With your Nikon's slow flash setting, I don't know.

    For the record, regular and rear flash can be used with any shutter speed, up to your camera's maximum synch speed. (That's why it's not the same as dragging the shutter.) So you can drag the shutter with any flash mode simply by putting the camera in shutter priority and slowing the shutter speed.

    Can any experienced Nikon shooters confirm exactly how slow flash works?

  • deck2 February 27, 2009 05:52 pm

    very nice tutorials, happy to get weekly newletter. slow sync flash is very usefull taking pics at parties, gives movement to photos

  • ben February 27, 2009 10:29 am

    I'm confused. My Nikon has regular flash, Slow flash, and Rear flash. This article only discussed two of those. Which one did it leave out?

  • Jeffrey Kontur February 26, 2009 01:28 am

    It's exciting to me to see people opened up to new possibilities with their shooting. To all those who confessed to be "afraid" of flash, don't be. The worst that can happen is that you take a picture and it comes out poorly. Using the flash will not break anything on your camera. Limit your experimenting to situations where you are not faced with once-in-a-lifetime shots. Soon, you will develop new skills and knowledge that can be gradually be applied to more important subject matter.

    To lils, yes you can use a built-in flash. You may be more limited in the results you can produce but it's certainly the easiest way to start.

    To Bob, I genuinely appreciate that you feel comfortable enough to dislike my photos. I mean that quite sincerely. I teach a lot of workshops and have written two books. So many people look at me as "the (exalted) expert" and do not feel comfortable forming their own opinions about what they see in my work. I dislike many of my own shots too. The ones posted here were not selected for their beauty but rather for their ability to demonstrate the concept.

    To Don, great question! As with many things in photography, the answer is most likely nothing more than tradition. Why is the particular size and aspect ratio of the 35mm film frame the de facto standard against which all cameras are measured? It's certainly not the only size or ratio ever used. It's arguably not even the best one. Front curtain synch became the default because it was first to market. back in the days when the technology didn't exist to make second curtain synch affordable, first curtain synch was the only option that existed. Thus, it became the "standard" against which alternatives are now compared.

  • 73Rocks February 25, 2009 02:12 pm

    Here is a couple of examples of how I like to utilize the 2nd curtain sync function on my Canon 430EX with my Rebel XTi when photographing high school sports:


  • Don February 24, 2009 11:04 pm

    So why is front curtain sync the default? The advantage of rear curtain sync is often discussed but I have never seen a clear statement of the advantage of front curtain sync.

  • Lou February 22, 2009 03:51 pm

    One thing to mention is that slow-sync flash is used with first or second curtain flash. Slow sync is simply using a longer than "normal" shutter speed. It can also be referred to as "dragging" the shutter; where you are using shutter speed to expose for some ambient light. The best example of 2nd curtain flash I know of is the image of a person carrying a birthday cake with lit candles. With first curtain flash the candle's light trails are in front of the cake and the person carrying it. Very unnatural. With 2nd curtain, the light trails from the candles are behind the cake, candles and person, creating a more natural feel and illusion.

  • thekevinmonster February 22, 2009 12:31 pm

    bob's got it right. the flash freezes the motion at the end of time, whereas with regular flash it freezes at the beginning.

    if your subject isn't moving, it won't matter which method you use, unless you're light painting and pick the wrong one.

  • Jim February 22, 2009 12:15 pm

    Once again it is soo great that digital Photography allows us to practice..practice. And it doesn't cost use a dime.

  • Bob February 21, 2009 04:54 pm

    Great explanation and graphics for the slow/rear/second curtain description.

    Your sample photos on the other hand leave a lot to be desired. While I can see what is happening in the photos, they are quite cluttered. I have seen other examples that show a car coming down the road at night taken with a time exposure using normal (front curtain) flash that have the flash firing first and then the light streaks continuing farther in front of the car. This gives a strange effect that makes the car appear to be going backward. However a similar photo utilizing rear curtain flash shows the streaks and then the flash fires at the end illuminating the car. This second or rear curtain example really illustrates what is happening.

    With only the headlight and tail light illumination creating the streaks and the car being considerably larger in the image than the streaks, it is easier for the novice viewer to understand the concept.

    Don't be afraid of flash, it can open up a whole new world of photography or enhance your outdoor photos by filling in the shadows. Check out for a lot of good tips on flash usage.

  • manilenio February 21, 2009 11:30 am

    definitely helpful bit of an article. im also one of the people scared of flash because it sometimes results in oily faces on people ;p @Tim, nice pictures - i like the second one, looks like a fireball

  • lils February 21, 2009 06:56 am

    same with Ilan, I'm scared of flash, only because I don't know much about it or practiced anything with. This article has definitely piqued more interest. For this cool 2nd curtain thing.. can you use a built in flash? I imagine it wouldn't reach as far as you like..?

  • Tim Collier February 21, 2009 04:28 am

    It's a simple technique but folk seem scared of using it - Flash seems to frighten people. I was out in the recent snow and using this technique to create some interesting and at times quite surreal images. The technique I used is explained here.

  • Ilan February 21, 2009 01:09 am

    That's cool. I actually never used it cause I'm "afraid" of flash :).
    Seriously, I bought a SB600 a few years ago and except a couple of trial and error shots, I never used it again.
    It seems to be that using a flash is a totally another world of photography, with different rules and different way of "thinking"/preparing for the shot.
    Articles such as this one help me to overcome my "mistrust" towards flash, thanks! :)

  • dcclark February 21, 2009 12:11 am

    "Slow Sync" (I've also heard "Rear Curtain Sync") is the effect used for those really cool National Geographic photos, in which you see (for example) a kid about to run out of the frame, with a long motion blur trail behind him. It gives a great sense of motion, while still letting you see the end result.