What is focal lenght?

Discussion in 'Photography Beginners' Forum' started by TwoRails, Nov 22, 2008.

  1. TwoRails

    TwoRails TPF Noob!

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    Can someone please explain, in simple terms, what focal length is and what it means in the world of photography? I've Googled it but the few I've looked at are way too scientific and or way too long of an explanation.

    Part B of the question is, how does it relate to focusing?

    TIA

    TR :)
     
  2. Helen B

    Helen B TPF Noob!

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    Focal length is the distance between a simple lens and the image it makes of a very distant object. The closer the object is to the lens, the further the lens is from the image it creates of that object. The lens can only bend the rays so much, so the more they are diverging when they arrive at the lens, the less they will converge as they exit.

    Just suppose that you are using 35 mm film. The size of the film image is 24 mm by 36 mm, or about 1 inch by 1-1/2 inches. Now imagine that you cut a hole of that size in a piece of card, and hold it in front of one eye. The effect that focal length has on field of view is like the distance you hold it in front of your eye - the closer it is (the shorter the focal length) the wider the field of view, and vice-versa.

    How's that for starters?

    Best,
    Helen
     
    Last edited: Nov 22, 2008
  3. TwoRails

    TwoRails TPF Noob!

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    Thanks Helen. I read your post a couple of times. I do understand the hole in the card example: I made a circle with my fingers and moved them closer / farther from my eye. I see how you have less field of view (proper phrase?), but that raises a question of "zoom." That is to say with my fingers further away, I see less but it's not zoomed in. I imagine that is the function of the lens elements to bring in the reduced field of view closer.

    So, is that to say a 50mm lens can "see" twice as much as a 100mm lens? ... But because the 100mm still fills the film / sensor, it appears larger?

    I'm I catching on?
     
  4. Helen B

    Helen B TPF Noob!

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    Yes, that's it. You should also be able to see that the larger the sensor/film (the larger the hole between your fingers) the larger the field of view for the same focal length.

    Best,
    Helen
     
  5. TwoRails

    TwoRails TPF Noob!

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    OK, so that would explain the "conversion factor" in lenses / cameras: Same lens + smaller sensor = 150mm when the lens is labeled 100mm (or the appropriate conversion as needed). I've heard of the term before but didn't know what it really meant until now! Thanks again :)
     

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