Let me preface this by first saying that the functionality of rear curtain sync flash on the V20 is limited due to it being a weak LED flash compared to even point and shoot cameras. So while it's a cool effect, don't expect it to work wonders on the phone. It is also only available in manual mode due to the nature of the technique.
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Rear curtain flash means that the flash fires at the end of the exposure, rather than the beginning. This is great for making light trails while still having the subject illuminated. I've thought about making a dedicated thread for this, but I'll sum it up here. Take this shot...
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This is straight out of the camera, unedited. To get it, I used a manual 15" exposure time at ISO 50 (I normally try to shoot the lowest ISO for the least image noise). I had the phone mounted to a tripod to eliminate camera shake. The light ring is me spinning a bundle of LED lights around in a circle. You'll also noticed I used the wide angle lens. This allowed me to get close enough for the flash to be effective while still capturing everything I wanted.
So how is this created? When using rear curtain flash, and light painting in general, you want to limit the amount of ambient light and wear dark clothing if possible to additionally limit the amount of light that reflects off you. Any light, including the reflected light off your body, hitting the camera sensor will start to essentially develop an image. The brighter your clothing and ambient light, the more any movement of your body can show up as ghosting like this (shot on my DSLR)
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By controlling the light and remaining still in my light ring photo, I created the light trails and then allowed the flash to freeze and light up my body. Had it been truly dark or pitch black, then I could move around all I want, for as long as I want during the long exposure and you'd only see the LED's still. In this case, the only time I'd be illuminated is whenever the flash fires with minimal ghosting
I would still suggest keeping unwanted movement to a minimum. Not only is the flash relatively weak, but it has a relatively long "on" time when it does fire. It's not a seemingly instantaneous POP like on dedicated cameras, so it's ability to freeze motion is not as effective. This is evident in my light ring photo if you look at my hand. It's a blur from spinning the LED's.