Adjustin KNEE

O

onmi

Active member
Does adjustin KNEE levels reduce chroma abberation?

I Have some issues with high contrast backgrounds, and that chroma thing appears if I put foreground unfocused and background focused.
And more I put it out of focus, the more visible that blue line comes (= thicker around edges)..

What options you PRO's use to reduce this?
 
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The knee doesn't affect chromatic aberration. Chromatic aberration is a lens property, there's no menu setting you can use to affect it. But then again, what you're seeing isn't chromatic aberration anyway, you're seeing an artifact of the CCDs being overexposed.

People get these two confused, but they're not the same. Chromatic aberration can cause color fringing, usually red or purple on one side, green on the other. But what you're seeing on the high contrast thing is not chromatic aberration, it's one of the CCDs clipping but the other ones not.

For example, when you have a situation where you have someone's head framed against a blown-out sky, you will probably see purple fringing. That's not because of the lens, that's because the green CCD is overloaded and clipped. The red and blue are still delivering signal, but now they're delivering it disproportionately strongly, and red+blue = purple. It's happening on the edges where there's averaged color information that isn't quite fully blown out.

People saw it a lot on the DVX because there's no adjustable knee in the cinegamma modes.

So to control that, yes the knee may help some, because it rolls off the CCDs to keep them from clipping. But the main thing to do is not let your background completely blow out. Balance your exposure better and you'll keep the situation from happening.

For true chromatic aberration, that's something that happens in the lens. To combat it you have to try changing focal lengths, or maybe reframe your shot so that it doesn't show up so prevalently.
 
Hey, thanks for clearing this thing out, I thought it was ca.. This thing seems also comin when shootin black & white lines or texts and it comes mostly on right side of footage.

I have tried to roll iris open->16 and put ND+roll iris, and those blue lines seems to be there all the time.. I havnt tried if that knee tweakin would help, but manually rolling iris open -> closed doesent seem to have any effect..
 
There's chromatic aberration, and there's clipping due to overexposure. If nothing is over 100 IRE in your shot, and you're still seeing color halos, that's more likely to be chromatic aberration (or, perhaps, inaccuracies due to the 4:1:1 color sampling).

If it only happens when the background is blown out, that would be an overexposure issue.
 
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