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Thread: Can I calculate the Focal Length of a cam lens?

  1. #1

    Default Can I calculate the Focal Length of a cam lens?

    I used 2 cams and the "3d glasses" effect in After Effects to create a simple video which requires red/cyan glasses. However, my settings were all guesswork.

    I would rather know what it will look like before I start filming.
    Sadly, I was not taught 'Optics' at school (40 years ago).

    After reading about some of the maths, I think the only information I don't have is the "focal length" of my camera. (Please hold back the sniggering; it is a cheap FLIP 1280*720 camera).

    How can I calculate the 'focal length'?
    I can measure the vertical and horizonal angles (fields of view), I can measure how close an object can be before going out of focus. I could measure the angles of distant and close objects.

    But. What can I do with the results? Can I calulate the Focal Length.

    The following 2 links may give a good idea of the area where I am currently lost:-

    Virtual Cinematography in After Effects 7.0 > 2.5D: Pick Up the Camera
    Calculating Stereo Pairs

  2. #2


    For any of that information to be of any use, you would need to know the size of the sensor!
    You are better off looking to the manufacturer for the 35mm equivalent focal length, as this has more meaning, especially in aftereffects.

  3. #3


    I am grateful for IanA's post.

    The spec appears to claim the Optical Sensor Size is 1/4.5", and the Lens Aperture is F/2.4 .

    I am principally interested (for amateur purposes only) in the changes in parallax for close and distant objects.
    Although the FLIP's lens is fixed, the physical distance between the two cameras can be varied.

    I am hoping to have a few simple formulas to apply, for example:

    a) If the two Flip cameras are separated by X inches, then how far 'in front' (or 'behind') the screen, will the filmed subject appear?

    b) If I add 3d AE objects; how can I calculate the required "z" values?

    Eventually, I might have to bite the bullet and run some tests and makes some measurements, but I would prefer to avoid that.

    Any maths teachers out there? Many thanks.

  4. #4


    It would appear that the active area of the Flip camera's sensor is :

    Active area: 3.58mm (H) x 2.69mm (V)
    however, the Flip site does not give an indication of the actual F/L of the lens, just a statement that the focal length is 35mm equivalent. !

    This could well mean that it is the equivalent of a 35mm lens in 35mm/full frame photography.
    As I a certainly no maths teacher, and have never seen/used a Flip, I would suggest you try 35mm as a starter for your focal length and see how the results come out. It does seem a sensible length for a fixed consumer camera's lens, being slightly wider than a standard lens.
    Good luck...

  5. #5


    This may or may not help. I'm so frustrated because I had a spreadsheet that I'd down loaded which I think would be just the job for you but I just can't remember where I got it from. If you Google some key words like focal calculator you may fine it.

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