Hello,
 
I have a project I am working on that had some problems.  The focal length is constant throughout the sequence but when I solved for the camera it calculated the wrong focal length of 78mm.  The result was that my whole scene was skewed.  So I manually entered 50mm as my focal length for the sequence.  Agian my scene was skewed so I consulted my camera manual.  The manual said the camera's focal length when zoomed all the way out, as the footage was recorded, was 28.5mm.  So I went back a changed the focal lengh to 28.5mm.  This time my scene was skewed in the other direction.  So I played with it a bit more and I got the correct solve with a focal length of 42mm.  My question is why?  May there be other errors in my camera solve that would appear to be corrected with a change in focal length?
 
Thanks.  That's all for now,
 
Audioman

HI  
 
your camera manual might be giving you the 35mm equivalant for your video camera  
 
You have to enter the film back width and height in mm also.
 
This is the atcual size of the sensor in the camera (the piece of film which would be exposed to light if you like)  
 
For example the camera i have been using has a film back of 8.8mmx4.95mm  and a focal length of 4.7mm (when zoomed out fully)
 
The camera manual gave me the 35mm equivlant and i was entering this in with a film back of 8.8mmx4.95mm which gave me odd resluts in Match Mover  
 
if this seems to be your problem then i should be able to tell you the correct film back width and hieght for you to use and the corresponding correct lens size.
 
Or maybe Ronald can put me straight,
 
But i think i am right  
 
Let me know

We are using a Panasonic PV-GS250 3CCD camcorder.  The specs in the manual go like this:
 
Focal Length: 2.85mm-28.5mm
 
Image sensor:  1/6-inch 3CCD image sensor
 
How do I figure this out in MM terms if these specs are 35mm equivalant?
 
Thanks for the help.  
 
 
 8)

hi  
 
I ve been trying to work out your ccd size in mm so you can enter this in to your film back width and height.
 
I think that the focal length is fine ( not the 35mm equivalant)  
 
http://www.dpreview.com/learn/?/Gl [...] zes_01.htm
 
 
your particlular sensor is not in the table  in the above link  
 
So i worked it out as 3.3mmx2.64mm enter this as your film back width and height and put your focal length at 2.85mm  
 
It is slightly hard for me to know if this super accurate as i dont know if your footage is 720x576  and the pixel astpect ratio etc
 
But ive worked the basis of it being 720x576 (1.25) with a pixel aspect of 1.067.
 
 
Let me know if this works. i hope it does

hey,
 
Thanks for the help, signet, I appriciate it.  Ok, I think I'm having the same problem that you were having with your camera.  I changed the film back width and height.  I solved the camera and it gave me a focal length of 12mm.  When I looked at the 3D points the scene was horribly skewed.  So I changed the focal length manually to 2.85 and applied a motion control to the camera.  The camera solve was not good at all.  I played around with the film back and height, but nothing helped it.  But when I go back to my 35mmx24mm film back settings, 42mm works perfectly for my focal length.  I am at a loss.  I don't know why MM is having problems with the different film back and focal length settings.  I'd like to know why, becuase then I can calculate for 35mm settings.  
 
I am working with my video in 640x480.  Not sure what my pixel aspect ratio is.
 
Thanks agian.

hi audioman
 
i have one mm-rule: do not enter any fixed values until you know they are 100% correct. if you don't know what your pixelaspect, focallength or filmback is, leave the calculations to matchmover. otherwise a single wrong parameter will screw up the 3d solve because all parameters are dependent to each other.
 
to get rid of the distorted perspective without knowing the correct focallength, use helper frames to get a perfect solve.
 
hope to help,
basti

Hey,
 
Normally I just leave the calculations up to MatchMover, but like I said in my first post, the solve came out skewed with an obviously miscalculated focal length.  I did all I could with my tracks without spending hours upon hours retracing them agian and maybe come out with the same results.
 
When we shot this footage we were rushed and did not think about helper frames.  We took a bunch of pictures, but they are all crap in terms of helping our sequence.  The sequence was a long angled dolly shot off the hood of a car.  All the photos we have are too close up.  Just trying to make do with what we have.  
 
I'm learning more everytime we shoot with MatchMover in mind.  There are about 3 or 4 things that I think we should have done everytime I go to track a new sequence.  Thanks for the help, basti.
 
Audioman

Hello,
 
As 'signet' suggested, the focal length is directly linked to the filmback size (so is the pixel aspect ratio).
 
Basically, the filmback depends on your camera, the focal of the lens you used. MatchMover might be able to find a solution with approximate values, but providing good estimation will save you most of the time
 
Anyway, if you have no way to figure them out, you can try to set up some survey points (with known 3D coordinates). This will impose a strong constraint on the focal. Using helper frames is also a good solution.
 
If you're still stucked, you can try to send us your project and we'll try to see how it goes.
 
Bye