CSE 590SS - Calculating the Focal Length

Since I couldn't wait to get the class camera, I tried using my own for the fridge panorama. This meant I had to guess at what the focal length might be. I came up with a method I call the "book, box, and ruler" technique. I'm not sure that it actually works, but the result was ok.

The first step is to find the optical center of the camera. The set-up is shown in figure A. Slide the camera towards the box, keeping the amount of book visible on each side of the box even. Stop exactly when the book disappears. This gives the geometry shown in figure B, and so you can determine the optical center relative to the camera, using the known dimensions of the book and box.

Now continue sliding the camera until the box exactly fills the field of view. Since you know where the new optical center is, you have the situation shown in figure C. The ratio of d/w is equal to the ratio of f/pixel width of your picture. For my camera, f = 689 for the shrunk-down 640-pixel wide images.

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