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Strange dark bars on lightning video

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Randy McWilson
Strange dark bars on lightning video
on Apr 9, 2011 at 2:43:43 pm

Here is an image from a frame of VIDEO (1080) of lightning that I shot today. Every bright flash of lightning caused dark bars like this one at the top. (shutter 30th) ISO 400, 10mm wide angle lens, Canon 7D.



Any ideas?

thanks
Randy

Eternity...don't miss it for the world.


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Daniel Hughes
Re: Strange dark bars on lightning video
on Apr 11, 2011 at 5:20:54 pm

Hi Randy.

It seems the 'dark bar' is just the sky you have photographed unilluminated, above more sky which is illuminated. The photograph seems to vertically represent a very short space of time.

I think it's your shutter to be honest. If you look at the speed your shutter moves, perhaps when that area of the image was revealed to the sensor as the shutter moved out of the way, the bolt of lightening was already gone.
You take the picture and as the shutter opens, the lighting flash is under-way, but when the shutter reaches around 4/5ths open the flash of lightening is already over, so the rest of the picture will be of the same scene, but after the flash has happened. This might be the reverse of what really happened, it really depends on which way your shutter opens.

A lightening flash is generally around 1/5000th of a second, so perhaps using a longer shutter speed, in good time before and after the actual flash (perhaps use BULB) and you will capture the entire flash within the time of the shutter being completely open.

And, of course, the lightening will blast a significantly higher amount of light that the dark stormy sky before and after it, so long shutter periods should work as long as you close your aperture a bit more or decrease ISO.

I could be wrong about it, but I think this is pretty much right.

Alternatively you could become a ninja and capture it at very very fast shutter speed...

I guess you'll have to wait for another storm to find out!
Well done for capturing this anyway, it must've took quite a while to get it!

Hope I've helped, and good luck next time!

Daniel


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Daniel Hughes
Re: Strange dark bars on lightning video
on Apr 11, 2011 at 5:28:35 pm

Oh, I just realised you said VIDEO! Whoops.

Well it could be the same thing I guess, but the shutter in video mode is the photo diodes turning on and off, obviously not the physical shutter release, so I could only really put that down to a firmware issue (the diodes of the sensor turning on and off at slightly different times, or in a fashion where it 'works its way up' in a mexican wave form, as opposed to all at the exact same time. Not quite sure about that though.)

You could try to fix it with editing software, where you could split the frame into two and composite either some of the bottom half of the video, and reflect it to the top or correct the colours in the upper half until they match, but this could result in intense grain..

If you're using 2.35:1 ratio you could even just leave out the top half of the frame completely also.

I think this problem may be similar to when you take video shots of CRT televisions, you tend to get a lot of madness and black bars.

Sorry I misread your post.

Daniel.


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David Bispham
Re: Strange dark bars on lightning video
on Apr 15, 2011 at 5:07:21 pm

I believe it is the rolling shutter issue with CMOS sensors
Please see...
http://en.wikipedia.org/wiki/Rolling_shutter



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Daniel Hughes
Re: Strange dark bars on lightning video
on Apr 16, 2011 at 1:21:45 pm

Is it the same thing as flash photography?
You see a video clip or whatever and the flashes seem to do the same thing...

Daniel Hughes
Amateur Writer, Director,
Director of Photography
United Kingdom


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Randy McWilson
Re: Strange dark bars on lightning video
on Apr 17, 2011 at 1:58:20 am

It looks like it is the CMOS shutter issue. Here is a snippet from wikipedia about it:

Partial Exposure. If a camera flash goes off in the shot, the quick nature of the flash may only be present for some of the rows of pixels in a given frame. So as an example, the top 1/3 of the picture may be brightly lit by a flash, while the bottom 2/3 of the picture is dark and unlit as the time it took for the flash to occur was only sequenced in a short amount of time as the CMOS imager sequences a frame. The difference between the two distinct parts of the frame can look odd. Similar problems can arise with emergency vehicle lights, some fluorescent lighting, or any extreme situation where very fast motion or very fast bursts of light are seen in the time between when the CMOS chip sequentially records a frame.

Wow. Now, is there anyway to get around it?

Eternity...don't miss it for the world.


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