Weird Flickering problem. Not compression related

[dizkoteck]

Hi there
First post and glad to be on the team. Ive shot with the Mavic a few times already and my latest trip at NYC Dumbo came back with some weird flickering. It is not H.264 related and the flickering are white bars going across the image so it is VERY noticable. Please take a look and let me know if you have any idea whats going on. THANKS!
Happened twice and always at that angle. Other side of the bridge no problem.

 

[msinger]

That's what's known as "prop shadows". You can prevent that from occurring by either always shooting with the sun behind the Mavic or by using ND filters.

 

[Plawa]

Shadows from your props..

 

[dizkoteck]

Really? Those lines look "electronic" tho

 

[msinger]

Here's an example of the same thing from another video:

 

[dizkoteck]

Thank you!

 

[Cslucas34]

Best way is to get some filters for your camera and that should eliminate that issue. The ND filters, higher numbers indicate darker glass. CPL is polarized and will help with glare from glass and water; should also help "see" into shallower water.

 

[Plawa]

Hold on, what's the logic here? Slower shutter speed smoothing out the dark/light areas of the shadows? That could actually work...

 

[Cslucas34]

Plawa,

I believe the filters are designed to eliminate certain wavelengths of light which causes interference on the recorded video. Similar to the reason if you record video of a computer monitor you see the flickering lines on the monitor on your video because the frequency of the display is similar to the frequency of the shutter. I could be way off here, but that's my understanding.

 

[Plawa]

You are way off but you inadvertently made an interesting point about ND filters possibly reducing prop shadow flicker..

 

[gnirtS]

As above, you're way off. An ND is a dark bit of glass. Nothing more. It blocks all light evenly. It goes nothing to specific wavelengths, nothing for contrast, dynamic range, highlights, it simply blocks light.

If your video is shot at a very high shutter speed (well above the 1/60 or so you need) then you may well "freeze" the shadow of the prop on the image. Obviously shooting towards the sun will force a high shutter speed.

By slowing the shutter speed you're effectively blurring the shadow to the extend it covers the whole image so isnt visible.

So its not the ND filters themselves that cure the problem, its just using whatever is needed to get that slow shutter speed. Sometimes NDs are needed for that, sometimes not.

There seems to be a lot of misunderstanding about NDs among mavic users - they're nothing special, just a bit of dark glass. They dont have magical selective blocking qualities.

Another possible cause if the SS isnt massively high is the prop speeds have become synchronised with the video so always appear in the same place. This could be cured by manually changing the shutter speed (and therefore iso) to get it back out of sync.

 

[Cslucas34]

Perhaps I should have clarified the differences in filters. ND or neutral density filters do indeed just darken the light which allows you to slow your shutter speed. The polarized lenses however do block directed refractive light and there are UV lenses that block the UV wavelengths as well.

 

[gnirtS]

CPL only works under very very specific circumstances. They need to be precisely adjusted prior to flight (rotated) and shots must be taken from that same exact sun-subject angle in the air. Any change in that they'll do little or nothing. They also only really work with the sun at 90 degrees to the lens, directly into or away from nothing will happen.

UV is a throwback to film days. No need for them on anything digital at all. Its just another piece of usually cheap glass to reduce image quality.