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  1. #11
    MattKing's Avatar
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    Quote Originally Posted by couldabin View Post
    But the light coming through the 8x10 iris is being spread over four times the area. ... Yes?
    No

    The light coming through the 8x10 iris is coming from a subject that is four times the area, and being spread over film that is four times the area, so it is a wash.

    It would only change if you changed the magnification to fill the 8x10 film with the same subject as previously filled the 4x5 centre (and then only if you were working at or near macro magnifications).
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

  2. #12

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    If the center (ie, the 4x5 film area) is the same exposure-wise for both lenses (and I would expect it to be if f16 means the same thing exposure-wise on both lenses), then we have to account for the all the extra light energy that falls outside the 6.5" circle when the 8x10 lens is used. The iris opening is the same for both lenses, so I'm not seeing how more light energy gets through the 8x10 lens.

  3. #13
    Steve Smith's Avatar
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    Quote Originally Posted by couldabin View Post
    But the light coming through the 8x10 iris is being spread over four times the area. If the intensity per unit area is the same with both lenses, the surely must mean four times as much light energy came through the 8x10 iris, even though it's no larger. Yes?
    Yes but the lens is also collecting four times as much light from the subject.


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  4. #14

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    In my effort to avoid the math and technicality of the optics of this subject I happen to have a 1982 version of Nikkor lenses for large format cameras. According to their chart format coverage shows the arc of the image circle at f22 for each of their lenses. They have lenses for different purposes such as general purpose, telephoto and wide angle. So for an example: their 150mm SW @f22 has an image circle of 10x12 inches and to get the same coverage in the W series all purpose lens you would need a 300mm @f22.

    I know this is not the answer you are looking for but the lens design is an influencing factor.

    http://www.jeffreyglasser.com/

  5. #15

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    Ah! But, of course! Don't know why that didn't occur to me at the outset ... but it's clearly the answer ... Thanks much.

  6. #16
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    If the aperture was a toll booth and we charged each photon a fare, then indeed, the 8x10" 210mm lens would pay a higher toll than the 4x5" lens, assuming it's area of coverage is much greater. So indeed, it's letting in a larger total quantity of light but not light of a greater intensity.

    This extra light is due to lens design and coverage alone, it really has nothing to do with the f/stop.

    Right?

  7. #17
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    Quote Originally Posted by Steve Smith View Post
    Yes but the lens is also collecting four times as much light from the subject.


    Steve.
    we have a winner...
    If you are the big tree, we are the small axe

  8. #18

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    Totally. It scares me just a little that I focused on the size of the circle of illumination (and the attendant energy) without thinking about that equally enlarged gathering area in front of the lens ... Duh.
    duane

  9. #19
    MattKing's Avatar
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    But the mental exercise involved in either:

    1) visualizing the reality and understanding the reasons for it; or
    2) visualizing the reality and figuring out a way to explain the reasons for it,

    is certainly all to the good!
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

  10. #20

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    Quote Originally Posted by couldabin View Post
    Totally. It scares me just a little that I focused on the size of the circle of illumination (and the attendant energy) without thinking about that equally enlarged gathering area in front of the lens ... Duh.
    You got the right answer! Just that one lens with the ability to gather light from a larger area and one doesn't.

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