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Matchmoving a dolly zoom with greenscreen

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Axel Nordman
Matchmoving a dolly zoom with greenscreen
on Feb 3, 2017 at 12:39:11 pm


I need some advice on how to go about to plan the visual effects, matchmoving and execution for this shot:

The challenge is that we are doing a dolly zoom (vertigo effect) with a green screen. We have to composite the two images together in post. We don't have enough budget for a motorized dolly. I'm the cinematographer and doing the VFX for this project.

Any help would be greatly appreciated.

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Mark Suszko
Re: Matchmoving a dolly zoom with greenscreen
on Feb 3, 2017 at 10:43:49 pm

I have never done this shot, but it seems to me all you really need is the foreground and background "plate" shots on separate layers. Then I would add the critical part of the effect, the lens distortion, in post, by keyframing changes to the virtual zoom/lens DOF settings. Should be able to mock this up in Apple Motion, or A.E. , whichever floats your boat.

Next, Todd will gently explain how everything I just wrote is bunk, and how he does it:-)

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Blaise Douros
Re: Matchmoving a dolly zoom with greenscreen
on Feb 7, 2017 at 9:57:01 pm

This is a Tall. Freaking. Order. Is there any way at all that you can get this in-camera?

Here's the deal: not only do you need to matchmove the camera movement, which is fine with a fixed focal length and a 3D camera tracker, but you need to replicate, exactly, the zoom movement of the lens, which is rarely linear--you also need to account for the big changes in lens distortion in a zoom like this. You might get away with cheating it by pulling a motion track from your greenscreen with tracking markers, and pulling Position, Scale, and Rotation keyframes, and compositing in 2D space vs 3D. But that's going to be complicated by the fact that you appear to have henchmen streaming in the doorway, blocking your tracking points. And with no motion control, you will not be able to shoot a clean plate for reference.

So here's what you do; you go shoot some tests. Throw a greenscreen with tracking markers at the end of the hallway, and see if you can get clean tracks from it, whether using a 3D camera tracker or a 2D PSR track. You don't want to bootstrap this one.

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Richard Herd
Re: Matchmoving a dolly zoom with greenscreen
on Mar 2, 2017 at 6:19:14 pm

In general, this is standard and relatively straight forward:

Normally with a zolly, it's very exact, and each take is written down accurately on set, noting all the relevant details from take to take.

We need to know (1) film size/sensor size and (2) fstop so the depth of field can be matched. We need to know the (3) focus distance goes from x-feet to x+dolly move (10 feet to 30 feet) and be sure to use the marks the manufacturer placed on the lens. The (4) focal length goes from 25mm to 85mm. And then you block and rehearse the action accordingly, and (5) how far away is the background.

Analyzing the board, the initial setup appears to have some serious problems. The scale of the scene is indeterminate. The green screen is 20x20 so it appears the camera is greater than 40 feet from the background, and shooting with a full sized sensor on an f/2.8 means the background is pretty soft, as you dolly the camera backward and zoom in, the bokkeh and background softness will get even more exacerbated. In effect, no actual lens in reality will do what those two thumbnails will do unless you have a huge amount of light, and I mean broad daylight, "sunny 16."

Enough technical mumbojumbo: From a storytelling point of view, almost always a zolly is used to reveal the character's emotions and feelings of being overwhelmed. This shot needs to be flipped to reveal the character's face so we the audience know what he's feeling at that moment.

Good luck!

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