Recently, I was working on this image, with the idea of illustrating the use of second curtain sync to get the movement in the right direction on the falling drops.
During the process I found an unexpected behavior from this feature. More after the link…
How 2nd curtain sync works.
First and second curtain sync differ on the moment the flash fire during a long exposure.
1st and 2nd curtain sync schema
First flash sync (default on most flash systems) is when the flash fires at the beginning of the exposure, therefore capturing or “freezing” any object in the frame. The shutter stays open for the remaining of the exposure, further capturing the events in front of the objective with the use of the ambient light. When subjects move during that time, they are registered as vague movement trails.
In contrast, with 2nd curtain sync, first the ambient light is captured and (in theory) just before the shutter curtain closes, the flash fires. In that way the subject is captured at the last possible moment before the end of the exposure. This renders any movement in its logical sequence and therefore images of moving subjects done using 2nd curtain sync look natural.
The name “2nd curtain sync” is due to the fact that in older cameras with a curtain shutter, the first curtain would open at the start the exposure and the second curtain could close at the end. Therefore “synching” with the second curtain would mean firing just before the 2nd curtain closed to end the exposure. In modern DSLR, the shutter is made of a series of thin blades, but the basic principle of opening and closing still remains the same.
2nd curtain sync issues
Going back to the image in question, with first curtain sync the flash freezes the drops and then the exposure continues, capturing the movement trail as they fall. This results in an unnatural image, as it looks like the drops are flying upwards.
Second curtain sync reverses this effect by firing the flash at the end of the exposure, therefore first capturing the movement trail and then “freezing” the drop with flash at the end of the exposure. This achieves the ‘comet’ effect with a natural trail behind the falling drop.
After many trials, I could not get the desired effect. In all images the visuals were showing the drops falling upwards regardless of the sync setting of the flash. I changed from a 430EX to a 580EXII to see if that was a flash limitation but the results remained the same. Only after making the exposure longer, the drops starting ‘falling’ in the right direction.
This lead me to the question: At what speed 2nd curtain sync actually starts?!?
2nd curtain sync Test
To answer that, I designed a more controlled experiment, with a single drop of water backlighted with a led array to ensure a clear and visible movement trail. (This setup is called ‘black field illumination’)
Then I proceeded to carefully go through all shutter speeds starting at 1/200 (X-sync on the EOS 5D MKII) and down by 1/3 stop each time.
Of course I was expecting that anything under 1/200 would show the effect of 2nd curtain sync. I was very surprised to discover that 2nd curtain sync actually only starts at 1/25s!
The following image table is an excerpt of the series, focusing on the transition speed at 1/30 compared to 1/25. Also note that when 2nd curtain sync starts, timing it not really at the end of the exposure, but rather in the middle of it.
Conclusion
I have never seen this “2nd curtain sync speed” in any Canon documentation. It’s good to know and have present to use this technique correctly.
For fast moving subjects, it’s a real limitation as 1/25 is just too slow. You have been warned.