View Full Version : undistorting a sequence shot with a zoom lens

05 May 2007, 10:23 AM
Hi, does anyone have any experience of undistorting a sequence that was shot using a zoom or how to do it?
The purpose ultimately being to track the plate for vfx work.
I know how to with a constant focal length but can't find any info regarding variable focal lengths.

The only way I can think of with out having some clever software is to film a lens grid with the same lens zooming throughout its range (thankfully we are using a arri lds lens so we can get fairly accurate focal length info), get a lens grid for each reasonable focal length step and then literally go through the shot undistorting it frame by frame. Obviously this is not very desirable.
Any help or info anyone could give me would be great.

05 May 2007, 04:46 PM
What info can you get from the Arri lds lens?

If you have focal-length info on a per-frame basis (ideal situation), then, if you can get a lens grid shot with as many points on the zoom range as possible, you can put together an expression that will move between the settings for each zoom level...

05 May 2007, 02:14 AM
if you can track 2 points in the footage, you can find how the scale of the image changes. And with a reference focal length at a specific frame you can calculate the focal length at any tracked frame (I know it's possible but I don't know the math for it... maybe it's just a simple ratio). Shake's Stabilize node can lock the scale of the footage for you, or match CG to the changing scale of the footage. You can also stabilize position and rotation (tilt, not pan) with 2 track points.

Even if your camera pans and you get perspective distortion, you can use 4 track points to stabilize it.

05 May 2007, 08:44 AM
If you have access to shake you could always use the "Lenswarp" node.

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05 May 2007, 08:45 AM
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