When should you use first-curtain vs second-curtain flash sync?
Asked 1/9/2011
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I often see second-curtain (rear-curtain) flash recommended for motion shots. When is it actually useful, and does it freeze motion better than normal first-curtain flash? Are there any downsides to using second-curtain sync all the time?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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Second curtain sync doesn't do anything more to freeze motion than first curtain. The reason to use second curtain sync is that the ambient light trails you get with a long exposure appear behind the subject (as the sequence is ambient first then flash) instead of in front, which looks a bit weird.
In the top image (first curtain sync) the headlight trail appears in front of the train, in the bottom image (second curtain sync) the headlight trails burn in before the flash fires, so they appear behind the train.
Can't think of a general disadvantage of second curtain sync (I use it all the time), except that not all cameras offer true second curtain sync. Canon cameras for example only send a signal on the first curtain. Second curtain sync is then achieved by sending the shutter speed to the Canon compatible flash which knows to wait the specified time before firing.
One reason not to use second curtain sync might be that it's easier to frame a moving subject if you concentrate on where the subject is when you press the shutter, rather than where it will be when the shutter closes.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
0
Generated from our catalog & community — verify before relying on it.
Second-curtain sync does not freeze motion any better than first-curtain sync. In both cases, the flash pulse is what “freezes” the subject.
The real difference shows up when you use flash with a slow shutter speed so ambient light records motion blur or light trails:
- first-curtain sync: flash fires at the start, so ambient blur/trails appear in front of the moving subject
- second-curtain sync: flash fires at the end, so blur/trails appear behind the subject
That usually looks more natural for moving subjects, which is why rear-curtain sync is often recommended.
If ambient light is negligible, there may be little or no visible difference between the two modes.
Downsides: there usually isn’t a major image-quality penalty, but not all cameras/flash setups support true rear-curtain sync well. Also, with some unusual high-power external flash systems that have very long flash duration, rear-curtain sync may not leave enough time for the flash to fully discharge during the exposure.
So: use second-curtain when you want motion trails to follow the subject naturally during a long exposure; otherwise standard first-curtain sync is fine.
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