A while ago I was trying to work out how to do something like slow-synch flash photography in video.
It’s easy enough, with the right camera, to shoot long exposure times in video, but the exposures are then far apart in time (at least the length of the exposure), and when played back the footage plays back too fast.
Eg if I shoot 1 second exposures at 1fps, when played back at 25fps the footage plays 25 times faster than normal.
This got me thinking, suppose I shoot at the standard 25fps with a shutter times of 1/25th of a second.
In one second the sensor is reading light falling on it for the entire one second, so what’s the difference between that and shooting 1 frame for 1 second? In both cases the camera is measuring how much light falls on the sensor over a one second period, it’s just in the former (at 25fps) you have additional information about when that light falls.
It seems it should be possible to combine the 25 exposures at 1/25th into a single image equivalent to one exposure at 1sec.
One way to do this is to have 25 layers of the same clip in my NLE, each starting one frame after the last, with the lowest clip set to 100% opacity, the clip above to 50%, the next to 100/3%, then 100/4% then 100/5% and so on.
Now for the first second of play back I have a stair step of layers coming on, but after that, for each 1/25th sec frame, there are 25 layers blended together equally representing 25 1/25th of a second periods.
I tried it, and it works!
That was shot with a sony RX100 at 25fps with a 1/25th sec shutter.
I did try it once before using Magic Lantern on a 5d2. The 5d2 usually has a minimum shutter time of 1/30 in video but Magic Lantern lets you go lower. It didn't work very well that time, possibly because of ML being screwy, but also because the subject matter was different, bright points of light on a dark background.
I think the way the video is exposed and then blended has some problems with clipped highlights - the math doesn't work properly once you get over 100% brightness.
Actually, in a slow synch flash photos there are effectively two exposure, the frozen flash exposure, and the longer ambient light exposure. What I've done with video is still only the ambient light part. It should be possible to do the flash part too, with a strobe light sychronised to the camera shutter and firing at 25fps - I'd like to figure out how to do that. Should be possible, but difficult, even if I just get a small strobe light, like they use in discos, and try to manually synch it to the camera. I expect synching and then drifting would be a problem.