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calculating focal length of a footage with zoom

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petcov petru
calculating focal length of a footage with zoom
on Dec 20, 2011 at 7:51:09 pm

Hello I apologize if I post this in inappropriate forum but I already posted in other forums and no replies so I'll give it a try here.

I want to track and get the right camera solve of a shot in Boujou with large zoom which I filmed it.I tried it but I can't get it right.
There's enough 3D points in the solved camera but they are positioned mostly on X and Y axis,just a few on Z axis and so in fact Boujou doesn't reconstructing the depth of the scene.
I tried it with automatic Assess Lens Distortion but the result is almost the same.From what I've found is to input the right value for the Focal Length but I don't how to calculate it.

I don't know for sure the exact display resolution at which my camera shoots.All I found about this is what I saw on the box in which came the Camera and there it says Display 1024X768 but I don't know what this represents - maybe is the display of the camera's LCD screen - if you know please tell me.I was setting the camera to shoot in the best mode.
The original files are MOD files and is Pal 1.09(i think).When I look at the file properties on my Windows7 it shows 720X576 and also when I import the video in After Effects it shows me the same display resolution.
But with that width it doesn't display the entire width of the video so I changed to Pal widescreen so now is 1050x576 PAL widescreen, 25 fps.
The camera is Canon FS100 if that matters.I'm very beginner so I don't know exactly what those numbers means but I can tell you that on the camera at the side of lens I see that is written 2.6-96.2 mm 1:2.0.
So to tell you how I have shooted the video - I was standing in the midlle of the street and I made a zoom from minimum to maximum on a building which close the street(like dead end street).
I aproximate the building beeing about 6-7 meters tall and maybe 20 meters wide.
The camera is hendheld and has some shake,I have not moved.

I don't know if I have to provide more information,but anyway that is all I can tell you at least for now.If you want to ask me something just go ahead.

So if anybody could help me I would much appreciate it.Thanks


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Walter Soyka
Re: calculating focal length of a footage with zoom
on Dec 21, 2011 at 2:09:46 am

If the camera is zooming, the focal length is changing during the course of the shot. You can't input a fixed focal length.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
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Darby Edelen
Re: calculating focal length of a footage with zoom
on Dec 21, 2011 at 8:11:57 am

To add to what Walter has said: the reason you're not getting any depth tracked on this shot is because there is no parallax.

Panning or zooming does not change the parallax in a shot, and the parallax is the only thing that a computer can use to estimate the focal length of a lens or depth of a scene. Parallax only occurs in a shot when the camera moves

This means you can't really solve this shot in any 3D tracker. On the upside, you shouldn't have to solve it in a 3D tracker because your shot doesn't have any (much) depth. A 2D tracker that supports position, rotation and scale should work for you. Mocha AE is a popular application that works well with AE. I don't know much about it though so you'll have to do your own research there :)

Darby Edelen


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petcov petru
Re: calculating focal length of a footage with zoom
on Dec 21, 2011 at 10:15:53 am

Thanks both of you but I really don't know what to say because I saw a tutorial(it's the 7 tutorial from the Boujou (5 in my case) Help menu - Using Focal Length Constraints,that one with the old taxi car - if you know it) and that shoot has a zoom.
So they input 4 focal length values at 4 different frames - Indeed the zoom is not that large as mine is and there is also panning too but Boujou solved it well.
From what I see(if i see well :)) the camera doesn't move.

Maybe I'm missing something,I don't know.


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petcov petru
Re: calculating focal length of a footage with zoom
on Dec 21, 2011 at 10:30:52 am

I forgot to tell you.They don't say where do they know these focal lengths values from.
Here is a quote from the tutorial:
"The focal length was varying throughout this shot but the value is known at four frames,including the first and the last"
And then is a table with these 4 values.

So how to they know these values?


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Darby Edelen
Re: calculating focal length of a footage with zoom
on Dec 23, 2011 at 3:46:18 pm

They most likely marked values on their zoom ring with tape and kept notes of these values.

Trackers can solve for panning shots but its a different kind of solve than one with parallax and will significantly hinder the solvers ability to estimate a focal length accurately.

Darby Edelen


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petcov petru
Re: calculating focal length of a footage with zoom
on Dec 28, 2011 at 12:27:39 pm

Thank you and sorry for my late reply.

Forgive my inexperience but can you explain a little more this:
"They most likely marked values on their zoom ring with tape and kept notes of these values."
because I don't really understand how this can help me whith my problem.How can I know for example the focal length for a certain frame

I just want to know how to calculate the focal length,at least for the first and last frame.


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