Vertical light bars when shooting in bright light at DVinfo.net

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Old September 19th, 2005, 02:19 PM   #1
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Vertical light bars when shooting in bright light

Hi !
I own an Optura 300 (MVX10i).
I just have noticed that when I'm shooting a scene with strong light, i get vertical light bars on my lcd. It is also recorded on tape. There are no such light bars in photo mode. Can someone explain, is it something that I should be worried about or is it a normal thing to happen ?!
Could a polarising filter (circular ?!) eliminate those bars ? Why are there bars in video but not in photo mode ? Perhaps there is linear polarising filter in the camera ? Why are those light bars verticaly oriented ?
Looking forward to your answer.
Thanks !!!

Marko Xorg

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Old September 19th, 2005, 02:25 PM   #2
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I suspect you're seeing a phenomenon called "vertical smear" which all camcorders have to some degree when there's a bright point source of light.

If you can use manual control make sure the shutter is locked at 1/60 for NTSC or 1/50 for PAL and try adjusting the iris to darken the picture and see if that helps. Otherwise I doubt you'll be able to do a whole lot about this, other than trying not to shoot into bright light sources.

Maybe someone familiar with this particular camera will have some more advice.
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Old September 19th, 2005, 02:49 PM   #3
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A lens shade can also help, depending on where the light is coming from. It seems to happen most when the light is:

a. bright with high contrast to the surroundings
b. above the lens
c. and 6 feet or more away from the camera.
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Old September 20th, 2005, 04:34 AM   #4
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Thanks for the input !
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Old September 20th, 2005, 02:21 PM   #5
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I can second Mike's shade/hood suggestion. I have a 60 and it seems far more prone to this than my previous camera (larger lens?). A hood or just a hand over the lens in the direction of the light works wonders...
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Old September 20th, 2005, 02:46 PM   #6
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A shade, while very helpful in many instances, would not correct the issue in this particular shot. This is an electronics issue inhearant in the operation of a CCD, not an optical issue.

What's happening here is that the brightest areas of the light, which are focused directly on the CCD, are actually exposing the CCD while it's is being read!

The way data from this CCD appears to be read from the top first.
After the first row is pulled, all the rows are shifted up by one so the top row now contains the second row of data that was exposed during the time the "shutter" was open. While this row is being read out, the light is still shining on the CCD (no phyiscal shutter like a film camera).

As the process repeats for each row, the bright light focused on the CCD leaves a little extra signal on the pixels for rows below the light in the frame as they pass by. This light doesn't spill upward, because pixels that started out above the light where moved away from the light source, not through it, on there way to the readout register.

Normally, you don't see the extra little bit of exposure, since readout happens so fast. With a bright light focused directly on the CCD, though, there is little you can do to avoid the streaking other than closing the iris, switching to a faster shutter speed or reducing gain, all of which will dim the surrounding image as well as the light.

Hope this helps.
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Old September 21st, 2005, 06:38 AM   #7
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Thanks for the detailed info Nick.

I'd love a technical breakdown of how film camera mechanisms (such as shutter, etc.) translate to digital cameras. For example, you mention that there is no physical shutter, so how is this done in the digital camera, or iris, is there a physical iris or it this another thing that is "simulated".

Can you recommend any websites or books on the subject?
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Old September 21st, 2005, 11:22 AM   #8
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Great post.
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