**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

I’m no optics expert, so I used a totally simplified model and came up with 2.8 m and 8.075 m for the centre tennis balls. My same model would predict about 4.075 m for the left-to-right distance of both, which doesn’t sound right, especially considering the balls look bigger in the second photo.

Got me!

OK, I think distance from Camera to centre ball (Pic B) is 2.16m.

That’s because a 12.29mm focal length gives a ~90 degree vertical angle of view and the ball is right at the foot of the view in the centre. So the ball must be 45 degrees below horizontal, 1.53m vertical.

So dist = 1.53/sin(90/2) = 2.16 (assuming the lawn is flat).

So you could get that position, and maybe the lower left/right, but for any other location all the photogrammetry I did tells me that you would need at least two photos – with a number of visual points in both – to be able to locate a single point in space.

Hi Guys!

Seems that Graeme got this very, very close.

Pic A: Distance is 2.90 m and Width is 4.03

Pic B: Distance is 8.00 m and Width is 3.68

I’ll send You the testdata!

Yikes! I was a mile out there! Well Maths and Physics were never my strong point.

I’ll get my coat….

Well…personally I had absolutely no clue before actually testing this. At least You had a theory!

You must have been assuming a 35mm sensor, Mark. The DSC-HX7V has a 7.75mm sensor.

I’ve put up a pictorial description of my simplified calculations on my dropbox account: http://dl.dropbox.com/u/2275900/Ulf%27s%20Camera.pdf

My model is extremely simple; lens design would no doubt have some effect on the actual optics involved.

Awesome! And it’s actually possible to understand!

Yes, I read on wikipedia that a digital camera’s sensor was made to be equivalent to 35mm. Guess that’s not always true!