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Photo Forum / General Photo Topics / General Topics / May 2008curious frame in lightning movie
There was an electrical storm nearby a few weeks ago. I set up my camera outside on a tripod and put it on movie mode. Within a minute or two there was a nearby lightning flash captured on the movie. When I played the movie back a frame at a time I noticed that just before the main lightning flash a faint purple tendril of light appeared to reach in front of the neighbouring houses and over our fence. At first I thought that this was an electrical phenomenon, maybe some sort of 'pre-strike', and that I'd narrowly missed being struck by lightning. On further inspection I noticed that the shape of the tendril matched exactly the shape of part of the main flash in the next frame, except it was transposed down in the frame. So, I'm presuming this is some sort of artefact caused by digital movie capture. However, I'm at a loss to explain it technically. I don't see how it could be an internal lens reflection of the main lightning flash as the main flash hadn't happened yet. And it can't be explained as a memory effect as it happened before the main flash. Does anyone have any explanation for these movie frames? Nine stills from the movie are here: http://tinyurl.com/3vvduo The seventh frame, just before the main flash, is the curious one. (The movie was taken on a Canon G9 with movie mode on 640 x 480 pixels at 30 frames/sec. AVI (Image data: Motion JPEG). Stills extracted using Graphic Converter v5.7X.) Cross posted to alt.photography and uk.sci.weather. > There was an electrical storm nearby a few weeks ago. I set up my camera > outside on a tripod and put it on movie mode. Within a minute or two [quoted text clipped - 26 lines] > > Cross posted to alt.photography and uk.sci.weather. Read this, it will explain what you saw. That is the way lightning works, it's s nice capture for your first try ! http://library.thinkquest.org/20698/data/light3.htm >> There was an electrical storm nearby a few weeks ago. I set up my camera >> outside on a tripod and put it on movie mode. Within a minute or two [quoted text clipped - 31 lines] > > http://library.thinkquest.org/20698/data/light3.htm Why would it appear considerably lower in Frame 7, to the extent of giving the false impression that it was between the camera and the building?  SignatureBlinky Killing all posts from Google Groups The Usenet Improvement Project --> http://improve-usenet.org Found 5/08: a free GG-blocking news *feed* --> http://usenet4all.se >>> There was an electrical storm nearby a few weeks ago. I set up my camera >>> outside on a tripod and put it on movie mode. Within a minute or two [quoted text clipped - 35 lines] > Why would it appear considerably lower in Frame 7, to the extent of giving > the false impression that it was between the camera and the building? It was between the camera and the building. It's called a "step leader" and it is a normal part of lightning. This explains it a liitle more: "So now we have an electrically charged cloud with ever-growing step leaders stretching out toward the earth in stages. These leaders are faintly illuminated in a purplish glow and may sprout other leaders in areas where the original leaders bend or turn. Once begun, the leader will remain until the current flows, regardless of whether or not it is the leader that reaches the ground first. The leader basically has two possibilities: continue to grow in stages of growing plasma or wait patiently in its present plasma condition until another leader hits a target. The leader that reaches the earth first reaps the rewards of the journey by providing a conductive path between the cloud and the earth. This leader is not the lightning strike; it only maps out the course that the strike will follow. The strike is the sudden, massive, flow of electrical current moving from the cloud to the ground." You can read more about it here: http://science.howstuffworks.com/lightning3.htm >>>> There was an electrical storm nearby a few weeks ago. I set up my camera >>>> outside on a tripod and put it on movie mode. Within a minute or two [quoted text clipped - 55 lines] > You can read more about it here: > http://science.howstuffworks.com/lightning3.htm Yep, it's just a step leader, and if it had been the first one to reach the ground he would have had one hell of a lightnig photo being that close to the camera ! >>>> There was an electrical storm nearby a few weeks ago. I set up my camera >>>> outside on a tripod and put it on movie mode. Within a minute or two [quoted text clipped - 52 lines] > follow. The strike is the sudden, massive, flow of electrical current moving > from the cloud to the ground." So there was a leader with its base in the courtyard, that was the first step of an identically shaped strike in another part of town? SignatureBlinky Killing all posts from Google Groups The Usenet Improvement Project --> http://improve-usenet.org Found 5/08: a free GG-blocking news *feed* --> http://usenet4all.se > So there was a leader with its base in the courtyard, that was the first > step of an identically shaped strike in another part of town? You (and Chris) are obviously correct. It's an artifact of some sort. SignatureAlan LeHun >> So there was a leader with its base in the courtyard, that was the first >> step of an identically shaped strike in another part of town? > > You (and Chris) are obviously correct. It's an artifact of some sort. See the article I just posted, Alan. I think it's an issue of sync (the lightning being equivalent to a horrendously large strobe <g>) and shutter travel, and the bolt just barely (and unevenly) spanning two rapid (this *was* movie mode) exposures. SignatureBlinky Killing all posts from Google Groups The Usenet Improvement Project --> http://improve-usenet.org Found 5/08: a free GG-blocking news *feed* --> http://usenet4all.se >>> So there was a leader with its base in the courtyard, that was the first >>> step of an identically shaped strike in another part of town? [quoted text clipped - 5 lines] > travel, and the bolt just barely (and unevenly) spanning two rapid (this > *was* movie mode) exposures. ...or, having just read the post from Chris that I missed, perhaps he's on the right track. Either way, it's not a leader in the courtyard; it's something to do with the interplay between how the camera works electrically/mechanically and the brief but very bright flash of the lighting. SignatureBlinky Killing all posts from Google Groups The Usenet Improvement Project --> http://improve-usenet.org Found 5/08: a free GG-blocking news *feed* --> http://usenet4all.se >>>>> There was an electrical storm nearby a few weeks ago. I set up my camera >>>>> outside on a tripod and put it on movie mode. Within a minute or two [quoted text clipped - 61 lines] > So there was a leader with its base in the courtyard, that was the first > step of an identically shaped strike in another part of town? Better take another look at photo 8. The lighning strike is between the house on the right and the tree just past it. See how the lightning is in front of the tree rather than behind the tree. >>>>>> There was an electrical storm nearby a few weeks ago. I set up my camera >>>>>> outside on a tripod and put it on movie mode. Within a minute or two [quoted text clipped - 65 lines] > house on the right and the tree just past it. See how the lightning is in > front of the tree rather than behind the tree. And is it between the camera and the building, as the leader is claimed to be? I think the strike, while surely short-lived, managed to span the opening of the shutter for two frames (this was movie mode, not 1/2 sec and then have a cuppa coffe before snapping the next 1/2 sec shot) -- just barely on the very tail end of Frame 7; far more on Frame 8. I can't explain why it was registered lower in 7 than in 8; might have something to do with shutter curtain travel if that uses a focal plane shutter; remember, a lightning strike is much like a strobe except you have no control of how it syncs with your exposure. SignatureBlinky Killing all posts from Google Groups The Usenet Improvement Project --> http://improve-usenet.org Found 5/08: a free GG-blocking news *feed* --> http://usenet4all.se > I think the strike, while surely short-lived, managed to span the > opening of the shutter That could explain the identical shape of the strike in F7 and F8 but not what it is half a frame lower. Bear in mind that a lighting flash is rarely just single flash, they are frequently several flashes close together bit still very fast individually. The other thing to remember is that this is an electronic camera, it doesn't have a mechanical shutter and we don't know if the electronic shutter was on of off. Can the OP confirm the status of the shutter? It may also be a strange artifact of the sensor related to the supression of overloads. Remember the vertical streak from highlights on earlly CCD cameras? Or something from processing after being readout from the sesnor. SignatureCheers Dave. >> I think the strike, while surely short-lived, managed to span the >> opening of the shutter [quoted text clipped - 11 lines] >of overloads. Remember the vertical streak from highlights on earlly CCD >cameras? Or something from processing after being readout from the sesnor. I have made a short animation to show the strike in F7 is just a transposed version of the F8 strike: http://img152.imageshack.us/img152/1095/animation3eu2.gif The camera exposes a frame for a set period of time, and then has to read the image off the sensor. If this is a CCD sensor it does this by shifting the whole exposed frame down a row at a time, reading the stored charge off the bottom row, and repeating this until the whole frame is read off. This happens so quickly that the incoming light doesn't noticeably affect the stored charge representing the image, as it slides off the sensor _unless_ it is bright enough, like a lightning bolt. The following link has a good explanation, although I can't be certain whether the OP's camera contains a frame transfer CCD or a full-frame CCD. http://en.wikipedia.org/wiki/Frame_transfer_CCD SignatureChris > I have made a short animation to show the strike in F7 is just a > transposed version of the F8 strike: [quoted text clipped - 15 lines] > > http://en.wikipedia.org/wiki/Frame_transfer_CCD Thanks Chris. That's an impressive explanation. The G9 has a Interline Transfer CCD. I also recall that movies recorded in very low light but containing small points of bright light can display the vertical smear mentioned in the wikipedia article. I'm a little disappointed that I haven't got a shot of a step leader. Maybe next time I'll fly a kite. Thanks to everyone who replied. >>> I think the strike, while surely short-lived, managed to span the >>> opening of the shutter [quoted text clipped - 31 lines] > > http://en.wikipedia.org/wiki/Frame_transfer_CCD I'll buy it, Chris. Thanks. SignatureBlinky Killing all posts from Google Groups The Usenet Improvement Project --> http://improve-usenet.org Found 5/08: a free GG-blocking news *feed* --> http://usenet4all.se >> I think the strike, while surely short-lived, managed to span the >> opening of the shutter [quoted text clipped - 7 lines] > doesn't have a mechanical shutter and we don't know if the electronic > shutter was on of off. Can the OP confirm the status of the shutter? Are you sure it does not have a mechanical shutter? I don't remember what make and model was used. SignatureBlinky Killing all posts from Google Groups The Usenet Improvement Project --> http://improve-usenet.org Found 5/08: a free GG-blocking news *feed* --> http://usenet4all.se > Are you sure it does not have a mechanical shutter? I don't remember > what make and model was used. Canon G9. I see it's really a stills camera but to see an image in the rear LCD screen any shutter is normally open... The exposure for stills may be made with a mechanical shutter but I doubt it will be for 30fps video. Such a shutter is expensive to make, delicate and noisy... SignatureCheers Dave. > Better take another look at photo 8. The lighning strike is between > the house on the right and the tree just past it. See how the lightning > is in front of the tree rather than behind the tree. No, the lighting is so bright it has flared out either due to the lens or the sensor or both. Look how it "eats" into the edge of the roof... It is behind the tree but has burnt out the detail of the foreground branches. SignatureCheers Dave. >There was an electrical storm nearby a few weeks ago. I set up my camera >outside on a tripod and put it on movie mode. Within a minute or two [quoted text clipped - 14 lines] >as the main flash hadn't happened yet. And it can't be explained as a >memory effect as it happened before the main flash. Looks like a CCD readout artefact - my guess is the instantaneous flash is bright enough to imprint itself on the previous frame as its being read off the sensor. Frame 8 is good though! SignatureChris >>There was an electrical storm nearby a few weeks ago. I set up my camera >>outside on a tripod and put it on movie mode. Within a minute or two [quoted text clipped - 20 lines] > >Frame 8 is good though! http://www.youtube.com/watch?v=nNLDdqCRjkE Go to 1:50 and you can see the same effect. SignatureChris > Nine stills from the movie are here: > http://tinyurl.com/3vvduo > The seventh frame, just before the main flash, is the curious one. They look like leaders to me. If you look up near the concentrated bit of lightning a little bit above the middle, and flick between frame 7 and 8, you can actually see the leader path that the lightning strike took. The leader that was rising from your side of the fence looks like it was never high enough to have any real chance of connecting to the downward ion path. SignatureAlan LeHun >>Nine stills from the movie are here: >>http://tinyurl.com/3vvduo [quoted text clipped - 8 lines] > never high enough to have any real chance of connecting to the downward > ion path. I disagree. It's an exact copy of the next frame. Even the swirl above the purple strike on frame 7 is a copy of frame 8. I suspect it's something to do with how the camera generates individual frames from the CCD and when it writes them to the AVI. |
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