Lens Length & Subject Size

Discussion in 'Photography Beginners' Forum' started by ElizaMM, Jan 13, 2019.

  1. ElizaMM

    ElizaMM TPF Noob!

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    I need a new zoom lens & I am trying to figure out what the increase in the size of a subject would be with a longer lens. For example, I was trying to photograph a woodpecker at 75 feet with a Canon 24-105. Subject much too small. Would the subject appear twice as large in relation to the outside measurements of the photo, in doubling the length of the lens? I am not looking for a suggestion for a lens specific to bird watching -- I just want to be able to compare several different lens lengths, with some understanding. Is there an algorithm? A chart?


     
  2. Ysarex

    Ysarex Been spending a lot of time on here!

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    The relationship is direct and so very simple. Assume at 75 feet using the 105mm lens the woodpecker fills 1/4 of the frame or with the full frame displayed on your monitor filling the screen the woodpecker measures 3 inches in front of you on your display. With a 210mm lens the woodpecker would fill 1/2 of the frame and measure 6 inches in front of you on your display. With a 420mm lens the woodpecker would fill the frame and measure 12 inches in front of you on your display. Double the focal length and double the size of the subject.

    Joe
     
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  3. Designer

    Designer Been spending a lot of time on here!

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  4. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    There are free apps for smart devices that you can enter your sensor format, focal length and distance to subject and it will return the area that will be captured.
     
  5. ElizaMM

    ElizaMM TPF Noob!

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    The relationship is direct and so very simple.
    I was hoping that that was the case--thank you. This makes my lens decision easier. I'm thinking a not-too-expensive Canon EF-S 55-250mm F/4-5.6 IS STM, although that should be another forum.

    I will also check out the suggested links and apps.
     

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