Thread: Fog increases FPN on ursa mini pro?

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  1. #11  
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    Can someone explain exactly what black shading is or point me to some resources to learn more? I'm curious. I read somewhere that what's happening is the camera is detecting fixed patterns in the gain across different sensor sites and adjusting for that. Is that right? Is what's happening that at different temperatures, each sensor site behaves a little differently and hence the fixed pattern might change?
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  2. #12  
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    I don't think either Red's "black shading", or BMs sensor calibration are black shading in the traditional sense. Black and white shading are about balancing uneven color/luminance at a sensor level. On a broadcast camera if you get a stuck pixel you auto black or power cycle it and that tends to get rid of it but that's a secondary function.
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  3. #13  
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    You can do it as many times as you like.

    You can do it when ever you feel like.

    Do it when the temperature changes.

    Do it whenever you do a black shade on any other camera that has a blackshade function.

    It's just not a blackshade that you're doing. You're doing a re-calibration of the factory blackshade. It simply address a drift that can happen with the 4.6K sensor (usually over weeks or months) from the factory calibration. It addresses a colour offset in the blacks.

    Notice how fast it is compared to a blackshade done on other cameras ? It's over in a second. It's not a blackshade.

    Ursa (and all BMD cameras) don't blackshade like other cameras. They do it like an Alexa. Notice there's no user blackshade on that camera ? You can't blackshade an Alexa.

    Because the camera maintains a very accurate operating temperature at the sensor level. Like an Alexa. They use active cooling, something RED and Sony don't do, to maintain a very precise operating temperature band.

    Instead of the camera's sensor temperature floating at whatever ambient temperature used, it's ACTIVELY and DYNAMICALLY cooled by a solid state refrigerator maintained in a very narrow + or - 1 deg band. If the sensor warms up, more CURRENT it applied to the solid state cooling so that it cools down the sensor MORE. if you're in a cool ambient temp environment, then LESS CURRENT is drawn to cool the camera.

    https://en.wikipedia.org/wiki/Thermoelectric_cooling

    "Photon detectors such as CCDs in astronomical telescopes, spectrometers, or very high-end digital cameras are often cooled by Peltier elements. This reduces dark counts due to thermal noise. A dark count occurs when a pixel registers an electron caused by thermal fluctuation rather than a photon. On digital photos taken at low light these occur as speckles (or "pixel noise")."

    Just because the camera gets hot on an Ursa or any BMD camera, doesn't mean the sensor is any hotter than when you first turn the camera on. It just means it's working the sensor refrigerator harder to MAINTAIN the same sensor temperature inside.

    This is EXACTLY how an Alexa works. Active and dynamic solid state cooling to maintain an internal sensor temperature so accurately, no other blackshade is needed other than the one done at the factory.

    RED's operating temperatures inside the camera vary wildly which is why you need to do all these black shades at different ambient temperatures. It's why their fans speed up and get noisier during long takes, but eventually you'll get a warning that the sensor is overheating. It doesn't mean it's literally overheating, it just means the operating temperature is too high to maintain the noise profile that's been calibrated for. Same for the 5D's etc and DSLR's. They don't actually overheat in a way that means the sensor melts. They just have to be cooler so the pictures don't get excessively noisy.

    You can blackshade an Ursa Mini as much as you like. It's just not doing what you think it's doing though.

    My comments about the lines being visible in LOG related to the UMP specifically and was actually addressed in a recent firmware update.

    From the latest release notes

    "• Fixed bug where scaling on 4” display caused monitoring image artefacts"

    I have shot many times with an ursa mini in fog and atmosphere and not had FPN problems. Many scenes in the currently airing series Queen Of The South have Ursa Mini shots in low light and with atmosphere.

    The only time I see FPN is when I'm trying to recover a very underexposed shot of dark and near dark information.

    JB
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    ^--^

    Ooooh...

    I frikkin' LOVE it when you talk dirty J.B.!
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    Oohhhhh SNAP! Somebody done got served
    Last edited by daydreamersproductions; 07-15-2017 at 02:43 AM.
    Go to my site to see my current Production and Post Production Gear
    http://www.daydreamersproductions.com/
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  6. #16  
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    Thanks for the detailed explanation John. That's very helpful
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  7. #17  
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    the Digital Bolex has to be warmed up for several minutes to stabilize. It has a DPC calibration routine for dead pixel compensation, but that does affect black shading and color tracking in deep shadows if it is not done properly.
    An interesting side note: The thermal envelope of the sensor was purposely calibrated to elevate the random noise floor above FPN. So you never see FPN under any conditions once the camera is at operating temperature.
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  8. #18  
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    That doesn't explain why my UM Pro had FPN out of the box from the factory, which inexplicably went away after running "sensor calibration."

    If it was actually calibrated from the factory, there shouldn't be FPN visible when it comes out of the box and has been running at operating temperature for a while. Running the calibration immediately remedied the FPN.

    I don't need to be "right" but I don't think whatever the engineers at BM told JB is 100% an accurate portrayal of what's happening with these cameras and their Process. Saying it over and over again doesn't make it true (same goes for anything I say) - I would urge anyone to test their camera and see what they see with their own eyes.

    Again - it's a very cool camera, and it makes great images. Just wouldn't count on anything to be calibrated right out of the box.

    If there is FPN visible in false color activated with the body cap on at the lowest ISO setting, I would not call it "factory calibrated."

    This FPN goes away if you "sensor calibrate"

    Final note - I understand there is a Peltier element cooling the sensor.

    Does the peltier cooler also warm the sensor if you're running below "operating temperature"

    FPN can appear when running BELOW operating temperature as well.

    This is why it's important to run your sensor calibration after the camera has come to operating temperature - whatever that means whether you're in the sun in The San Fernando valley or in the arctic tundra surrounded by ice. You can get FPN from being too hot OR too cold. There's a narrow window of "correct" temperature for these sensors.
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  9. #19  
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    Quote Originally Posted by DanKanes View Post

    Does the peltier cooler also warm the sensor if you're running below "operating temperature"
    Yes.

    It works both ways.

    The camera needs to be on for 60 seconds for everything to come up to optimal temperature. In some ways it's better to leave your camera on over a day rather than turning it on and off.

    JB
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  10. #20  
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    Thanks JB - wanted to let you know I've admired your work for a long time and think you're a great guy.
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