How to keep the Girls sharp combined with the lighttrace effect?
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For this kind of a shot, it doesn't matter if the flash is first or second curtain because there is no direction to the movement. (ie, the phone could have started on the left or the right side and we'd have no idea of the difference). What does matter is that you have a dark environment so that only the light trail is normally exposing. You then take either a long exposure or a bulb exposure (which keeps the shutter open as long as you hold the button down).
The flash exposes the people and since it is dark, they don't continue to expose much, but the light trail from the bright screen exposes quickly and thus you see the light trail but not much else that was exposed during that part of the shot. It also probably took a fair bit of practice (or luck) to get it to come out that well as typically the light trails would be less even.
It's also possible that they faked the effect in Photoshop since that is one heck of a smooth circle. It's certainly possible to get that effect in camera, but they seem to have done a remarkably good job in many aspects, so either they did a lot of time and patience to get it just right or they blended it in artificially (in part or full).
To produce it in post, using an additive blend would properly mix the light sources in the image with the light source of the rainbow such that the sum of the brightest spots would shine through. Any tool of choice could be used to produce the actual arc.
This is how I think It could be done.
1. Set camera to bulb mode. So that the shutter stays open as long as the shutter button is pressed.
2. Set flash to fire either at the first / second curtain.
3. Press the shutter button when the people start to move the light source and release it once the movement is complete
As you realize, I've not specified what ISO or Aperture is needed... this is where you need to experiment.
This would seem to be a most UNlikely result if done "live"
Bacground over foreground and no apparent mixing even at boundaries.
This very strongly suggests it was done in post processing.
Also, the smooth radius change curve does not feel likely to be "natural". When swept in large arc the arm not only changes length as it moves but also the rate of change of length with position & times varies between locations as the arm geometry changes.
I have some swept arm arc time exposure of people waving "sparklers". I'll post one when/if they show up to allow comparison of arc shapes.
Sparkler arc - full 'circle' in this case.
More circular that I'd recalled but not the perfect arc of the photo above.
Also does not include any of the background hanging over the foreground as theirs does :-).
No post-processing additions in this one :-).
Chinese New Year, Ningbo, China: