I need to use Focallength but it’s tricky.
In my previous blog-post I confessed that I never really quite understood all the technical lingo with SLR Cameras. Today I realized that positioning pictures in 3D with FME still demands some knowledge about photo-lingo. Good thing I studied remote sensing 15 years ago then… So understanding what focallength means should be no problem – right? Just studying a bit of Wikipedia about the subject and we should be fine.
So I did. It did not help. I still can’t figure out how to use this in theory.
Why the interest in focallength then? Well – as far as I can see it is the only Exif-tag I can read that has information about “the distance to the picture”. What I mean is the distance to where I would position it in the 3D space.
So today I decided to experiment and gather the data I needed to understand this. And I did. However I have not fully analyzed the result. I will publish this shortly. However I will use this moment to announce a challenge.
Here is the challenge for You Camera nerds who knows all about focallength
1. I mounted my Camera on a Tripod in my garden. 1.53 m above ground pointing straight ahead.
2. The camera has these detailed technical specifications (For instance the Focal Length, f= mm 4.3-42.5 )
Below are two pictures.
* The first picture has the Focal Length 4.25 mm.
* The second one has Focal Length 12.29 mm.
(Note! ignore the big red and blue ones. It’s only the three Tennisball that should be used)
And here are the questions:
A. What is the distance (in metres) from the camera to the middle Tennisball for each picture ?
B. What is the distance (in metres) from the leftmost Tennisball to the Rightmost Tennisball for each picture?
If You post a comment with an answer I will give You the full testdata in Excel (gathered with FME) !

Picture A above
* What is the distance to the middle Tennisball from the camera? Focal Length 4.25 mm
* What is the distance from the leftmost Tennisball to the Rightmost Tennisball ?

Picture B above
* What is the distance to the middle Tennisball from the camera? Focal Length 12.29 mm
* What is the distance from the leftmost Tennisball to the Rightmost Tennisball ?
/Ulf Mansson