Difference between focal length and focussing distance???

Discussion in 'Photography Beginners' Forum' started by Pband, May 13, 2009.

  1. Pband

    Pband TPF Noob!

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    Dear friends,
    Can anybody please explain the difference between the focal length and focussing distance?
    I understand that focal length is the distance of the focal plane from the lens axis.
    I believe focussing distance is the distance of the subject from the lens axis.
    So, I think that using a focal length lens, the distance of the subject from the lens must be kept fixed in order to keep it in sharpest focus. Then how can one take photos of subjects in different distances using a fixed length lens?
    I know, I am way confused. Please help me.
     
  2. Ptyler22

    Ptyler22 TPF Noob!

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    Focal length, is the "zoom" whether it be 50mm or 200mm...
    Focussing distance is how far away the object is that is being focussed on. Say I'm taking a portrait of a person standing 10 feet away from me. The focussing distance is 10 feet.
     
  3. smn_xps

    smn_xps TPF Noob!

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    the focal length of a lens, or an optical system, is a property of the lens. it is the distance form the center of the optical system to the location where the film would have to be placed so that parallel light coming from a source will be in focus. by changing the distance between the film and the lens you can cause the system to focus on sources of light (the subjects of your photos) that are different distances away.

    if you have a magnifying glass you can do a simple experiment to demonstrate this: in a dark room light a candle and use the magnifying glass to focus an image of the candle on some paper. this is a basic camera, sans shutter and film. now move the candle further away and refocus by moving the lens. this is a lens of a fixed focal length that is being used to focus light onto some film (paper). you moving the lens is more or less what happens when you turn the focus ring on your camera. in this case you are not changing the focal lenght of the lens (you can't its made that way) but you are adjusting the focus.

    a zoom lens would be like changing your magnifying glass for one with a different focal length, the size of the image on the paper will change.

    hope this helps
    jerry
     
  4. Dwig

    Dwig TPF Noob!

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    Focusing Distance - this is the distance at which the image is focused. It is usually measured from the image plane (film or sensor) to the subject which is at the point of sharpest focus. Under some special conditions, it may be measured from the front of the lens to the subject, but when this is done it should be stated that this alternate measuring method is used.

    Focal Length - in common terms, the distance from a point in the lens system to the image when the subject is at infinity. Check out this page:

    Focal length - Wikipedia, the free encyclopedia
     
  5. Pband

    Pband TPF Noob!

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    Thank you very much friends. Specially Jerry (smn_xps). Jerry's explanation helped me a lot. Thank you very much.
     
  6. smn_xps

    smn_xps TPF Noob!

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    no problem,
     
  7. Dao

    Dao No longer a newbie, moving up!

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