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confused ? focal length ?

  • 11-07-2007 2:52pm
    #1
    jackdaw
    Closed Accounts Posts: 1,788 ✭✭✭


    Ok so focal length .. the smaller it is the wider the angle correct ?

    well this photo

    http://www.flickr.com/photo_exif.gne?id=703228023

    says it has a focal length of 7mm .. so it should be wider ?

    almost distorted on the sides at 7mm ??

    or is this because it's not an SLR ?


Comments

  • #2
    Morlar
    Closed Accounts Posts: 9,273 ✭✭✭Morlar


    jackdaw wrote:
    Ok so focal length .. the smaller it is the wider the angle correct ?

    well this photo

    http://www.flickr.com/photo_exif.gne?id=703228023

    says it has a focal length of 7mm .. so it should be wider ?

    almost distorted on the sides at 7mm ??

    or is this because it's not an SLR ?


    That was taken with a compact not an slr so I think it records the focal length differently. If it had been taken with an slr at 7mm it would have somewhere around a 180 degree field of view with a lot of distortion.


  • #3
    DaireQuinlan
    Registered Users, Registered Users 2 Posts: 6,713 ✭✭✭DaireQuinlan


    wellll, strictly speaking the focal length of a lens is
    "The distance between the film plane and the focal point (optical centre of the lens) when the lens is focused at infinity."

    Its normally given in MM, and yes, the bigger the distance, the narrower the field of view of the lens (and the more tele-like it gets). Everyone, having mostly used or been accustomed to the 35mm format for the last umpty years is familiar with the range of focal lengths common to that format, IE 20mm is pretty wide, 50mm is normal, 600mm is a long telephoto etc etc.

    However this changes according to the size of the surface onto which the lens image is projected (or crop-factor as the digital afficionados call it). Stick a frame the size of a piece of 35mm film 7mm behind that lens and yes, it would be super wide and horrendously distorted. In reality it wouldn't even cover a fraction of the frame.

    Mini digicam sensors in those cameras however are about the size of the nail on your little finger so even thought the focal length of the lens is 7mm the crop factor makes it appear larger. In the specs for the camera it probably gives it as 28mm-80mm (35mm equivalent) or something similar.


  • #4
    amcinroy
    Registered Users, Registered Users 2 Posts: 349 ✭✭amcinroy


    Yes, it can be confusing.

    Focal length is a lens parameter that is independant of the system it's mounted on.

    The resultant angle of view recorded is affected by sensor size. That's why a 20mm lens behaves differently on full frame camera compared to APS cameras.

    But this one is still a 7mm lens, whatever it's mounted on.

    Andy


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