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Thread: Bell extension factor vs. effective aperture

  1. #11

    Join Date
    Nov 2017

    Re: Bell extension factor vs. effective aperture

    Quote Originally Posted by lipi View Post

    In my case the aperture diameter can be determined.
    Attachment 238885
    that's cheating! :P

  2. #12

    Join Date
    Aug 2013

    Re: Bell extension factor vs. effective aperture

    Quote Originally Posted by Drew Bedo View Post
    As you extend the bellows, the aperture stays the same. The aperture is related to the focal length and the dimeter of the lens. These do not change.

    What changes is the amount of light that falls on the area of the format. As the lens moves away from the focal plane, the image circle expands, which spreads the available light out. The intensity of the light falling on the area of the format (the film) drops off inversely with the square of the distance.

    See for an explanation of the inverse square law.

    Look at for an discussion of f-stops.
    You're absolutely right! That is why I use the "effective aperture" phrase...

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