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    Rainbow-like Moire?
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    Hi,
    The following link shows an example of a kind of rainbow moire pattern I shot the other day on the C300 while conducting some lens tests. I've been shooting with the camera constantly under numerous less-than-favorable conditions and this is the only instance where I've seen this problem occur, so please treat this as a very unique case-study. But this may be something to be aware of in case you decide to bring this camera into a space with a lot of stained glass, like a church.

    The rainbow pattern that emerged is not the result of my ISO settings, my user settings, or anything else, but I do believe it's something that the sensor is doing. The footage was shot using Canon Raw (user settings totally disabled), utilizing multiple ISOs, multiple lenses, multiple ND settings, imported as Prores or MXF, all with the same results, which is the sort of localized rainbow moire pattern in certain areas. My hunch is that it has something to do with the quality of light coming through a stained glass window directly behind what I was shooting, but that is just a non-scientific hunch and nothing more. One other thing: NONE OF THIS SHOWED UP ON THE MONITORS AS I WAS SHOOTING IT. And I tried it on both the Canon c300 monitor and a separate SDI Monitor just to be sure it wasn't the resolution of the c300 monitor that I was up against.

    The file is 35 meg, it was converted to h264 for viewing purposes but this pattern in the video is NOT an artifact of that compression. Showing this on vimeo doesn't make it as clear as what I'm seeing off the camera in its raw state so I had to post it this way instead.

    Anyone have any ideas? Anyone experienced this before? It's quite a difficult if not impossible thing to remove in post, so this is why I'm asking.

    Thanks, Mike

    http://michaelpalmie...ainbowMoire.mov


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    Senior Member cowpunk52's Avatar
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    Interesting. I'd say you just nailed the moire frequency with the camera. As Alister Chapman said in his review of the camera when he pushed the limits of fine detail, "Now before everyone runs off in a panic, lets put this into perspective. The F3′ aliases, the Alexa aliases as do most single chip cameras. This is certainly no worse than an F3 and is right at the resolution limits of the camera, so your not going to see it very often. It takes a very fine, high contrast pattern, in sharp focus before you’ll see this kind of thing."

    I think it has a lot to do with the exceptionally high contrast on the slightly off-level horizontal lines. I'm sure you noticed already, but in the video, as you add ND and/or stop down, the moire begins to disappear.

    If you have access to the same spot, I recommend to try your sharpness dialed all the way down to -10 and see if that helps alleviate the issue.

    *as a point of note, there is actually no Canon RAW. I'm assuming you mean the C-Log lock. If I'm not mistaken, at C-log lock, the sharpness is set to 0. This could definitely be exacerbating the problem, especially if you're using a particularly sharp lens at optimal aperture (read Alan Roberts C300 evaluation for more info on this). Try again using C-Log gamma but sharpness dialed down.
    Last edited by cowpunk52; 03-08-2012 at 09:59 AM.


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    Senior Member cowpunk52's Avatar
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    Also, out of curiosity, what is the resolution of your SDI monitor? If less than 1080, does it have a pixel-to-pixel zoom? I try to make sure I view at 100% all sections of high contrast, fine-pattern frames with any single-sensor camera to make sure I'm not getting moire or aliasing issues. If it's coming from the sensor, it will show up in a monitor at 1:1.


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    Senior Member J Davis's Avatar
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    Mike, I've developed a technique to remove color moire (not luma moire) in post. I developed this based on photoshop techniques I used when removing CA when I was a retoucher at Getty. Details at this link
    http://jdmax.com/how-to-fix-chroma-m...or-canon-dslr/
    J.Davis
    jdMAX.com


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    Quote Originally Posted by cowpunk52 View Post
    *as a point of note, there is actually no Canon RAW. I'm assuming you mean the C-Log lock. If I'm not mistaken, at C-log lock, the sharpness is set to 0. This could definitely be exacerbating the problem, especially if you're using a particularly sharp lens at optimal aperture (read Alan Roberts C300 evaluation for more info on this). Try again using C-Log gamma but sharpness dialed down.
    Yeah, sorry, I meant C-log lock. But dialing down sharpness sounds like a plan for future avoidance.

    The monitor I was using was a smallHD 8.9" so not 1 to 1 in terms of pixel mapping, which means you're right, it could have been caught on a giganto 1:1 HD monitor on set, but that's not anything any of us ever has in the field with a doc.


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    Wow, cool, thanks for this J. I'm curious if this would work for the kind of insanely sharp patterns I was seeing - I was going to note in the original post that this didn't feel like the kind of soft moire aliasing that you can sort of fix in post in an easy way -- although i've not tried your technique, it would be interesting to see if it works here. This pattern was extremely harsh in a localized area, seemed like a total roto job to me.


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    Senior Member J Davis's Avatar
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    I'm 100% sure my technique will fix this.

    Also I'm sure you will be startled at how effective it is !



    BTW this looks like chromatic aberration from a lens, not moire from a sensor.
    The technique I described works for CA (chromatic aberration) as well.
    J.Davis
    jdMAX.com


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    Senior Member cowpunk52's Avatar
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    Quote Originally Posted by Michael Palmieri View Post
    Yeah, sorry, I meant C-log lock. But dialing down sharpness sounds like a plan for future avoidance.

    The monitor I was using was a smallHD 8.9" so not 1 to 1 in terms of pixel mapping, which means you're right, it could have been caught on a giganto 1:1 HD monitor on set, but that's not anything any of us ever has in the field with a doc.
    Some monitors employ a pixel-to-pixel crop function for viewing sections of the frame at 1:1 when necessary. I know my TVLogic 5.6" does this, but I don't know if the SmallHD offers any kind of image zoom. If it does, this would a very smart feature to be aware of. I use it almost all the time on the TVLogic.

    I believe the magnification function on the C300's LCD is 1:1 as well.*

    *nope, it's not. It's a 2x magnification.
    Last edited by cowpunk52; 03-08-2012 at 10:59 AM.


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    Senior Member cowpunk52's Avatar
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    Quote Originally Posted by J Davis View Post
    BTW this looks like chromatic aberration from a lens, not moire from a sensor.
    The technique I described works for CA as well.
    Good point, I didn't even think of that!


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    Senior Member Philip Goetz's Avatar
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    Fabric sample test:

    http://vimeo.com/37640157


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