FOUND ON 49 | THE STORY OF JIM WALMSLEY
Video Statistics and Information
Channel: Summit Love Productions
Views: 407,805
Rating: 4.9176526 out of 5
Keywords: jim walmsley, western states endurance run, western states 100, hoka one one, found on 49, western states
Id: DZb7jBYL9y8
Channel Id: undefined
Length: 48min 40sec (2920 seconds)
Published: Wed Jun 21 2017
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A combination of the light and the camera.
Many flashlights regulate brightness by a process called pulse-width modulation. The light is flashed on and off, and the brightness is controlled by adjusting the ratio of time spent on to time spent off. When done well, this is invisible to the naked eye, but it's still visible on camera under the right conditions.
The reason it shows up on camera is due to the rolling shutter effect. I don't know enough about that to explain it well, but it's basically an effect due to the way digital cameras take photos.
From the start where I linked the clip to about 35:55 the headlamp being used cause these weird lines on the screen.
Is this issue caused by the camera or the light?
I know Black Diamond/Petzl lights are undesirable. I run a Skilhunt H03. But as I am starting to do more filming on the trails I am curious what is the cause of those lines.
Both. It's a function of how the light works and how the camera is set. This always occurs, we just can't see it with our eyes (or at least perceive it consciously).
Most camera sensors read out linearly, meaning that the whole image isn't expose at once across the sensor, but more like a fire-cracker chain going off within whatever shutter speed the camera is working at.
Sometimes these shutter speeds coincide with the PDM of lights- Canon's DSLRs include tech called "Flicker! Detection" that senses when your shutter speed is going to "line up" with the so-called flicker-rate of the light source, and warns you, so you can adjust your shutter speed so your exposures won't vary. In video, this reads out as lines as we see here, and you may also have seen it on your local TV news channel when they film someone at a computer screen or TV monitor that has the same effect.
This also sorta-happens with rotors of helicopters and planes- I think we've all probably seen one of the few popular videos of this "hovering helicopter" effect.