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  1. #41

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    Quote Originally Posted by Q.G. View Post
    Oh dear me...
    No, i'm not!

    You say you have read the explanation i gave several times, and you still come up with that line?

    It's like pulling teeth... It really is...
    All due respect, but my initial reading of your first half-dozen or so responses caused me to believe that you were. Your position only became clear to me in the last few posts.

  2. #42
    Diapositivo's Avatar
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    I finally got it.

    I didn't take into account that f/number in exposure is not an "absolute aperture" but an aperture "relative to the focal lenght" (not by chance is named f/) and that the actual aperture of the diaphragm, for a 300m set at f/8, is higher than for a 50mm set at f/8. The higher absolute aperture of the tele lens compensates for the different angle of light capture, so that the exposure is the same.

    Q.G. tried to put me in the right direction when mentioning different lens diameters, but I did not put this in relation to the same f/# and so the same exposure for the two cases. And when Q.G. answered to my question confirming that the light from distance was less, I was even more confused because practical experience shows that exposure does not change.

    Lux facta est.

    Fabrizio
    Fabrizio Ruggeri fine art photography site: http://fabrizio-ruggeri.artistwebsites.com
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  3. #43
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    QG, please explain why my wall reads an EV of six, regardless if I place my spot meter one foot away or twelve.
    That's just, like, my opinion, man...

  4. #44

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    Do you really want AN explanation, or an explanation from QG?

  5. #45
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    Quote Originally Posted by Tim Gray View Post
    Do you really want AN explanation, or an explanation from QG?
    Yeah, it should clear it up for everyone.
    That's just, like, my opinion, man...

  6. #46

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    Quote Originally Posted by JBrunner View Post
    QG, please explain why my wall reads an EV of six, regardless if I place my spot meter one foot away or twelve.
    Because you painted them a rather dark colour?

    Because the spot sees more of it when it moves away.

  7. #47
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    Something more to argue about:

    Point sources fall-off as distance ^ 2
    Line sources as distance
    Planar sources don't fall off ...

    So what's the fall-off inside a spherical source?
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
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  8. #48

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    Quote Originally Posted by Nicholas Lindan View Post
    Something more to argue about:

    Point sources fall-off as distance ^ 2
    Line sources as distance
    Or a collection of adjoining point sources, the light of each falling off as distance ^ 2.


    Quote Originally Posted by Nicholas Lindan View Post
    Planar sources don't fall off ...
    They do, as a collection of adjoining line souces.

    The principle remains the same. The math gets complicated.


    Quote Originally Posted by Nicholas Lindan View Post
    So what's the fall-off inside a spherical source?
    Does the source get brighter when its diameter gets bigger?
    Else it's a complicated instance of the inverse square once again.

  9. #49

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    Quote Originally Posted by Q.G. View Post
    They do, as a collection of adjoining line souces.

    The principle remains the same. The math gets complicated.
    You treat the line source as a line or a collection of points, there's no real difference. Actually the math is quite easy. I'm not sure what you are trying to prove with this kind of statement.

    If you really insist on calling a line a collection of points, then go right ahead. Most of us, casually and mathematically, have a name for a collection of points: a line. As was stated before, inverse square law stems from essentially geometric arguments.

  10. #50

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    Quote Originally Posted by Tim Gray View Post
    You treat the line source as a line or a collection of points, there's no real difference. Actually the math is quite easy. I'm not sure what you are trying to prove with this kind of statement.

    If you really insist on calling a line a collection of points, then go right ahead. Most of us, casually and mathematically, have a name for a collection of points: a line. As was stated before, inverse square law stems from essentially geometric arguments.
    What that statement is trying to show (if it is trying to show anything) is that this inverse square thing is not such a strange and exotic thing.
    There's no real difference indeed, whether point or line, except that you don't calculate the effect of a change in distance from subject to one point light source, but to many. Each behaving in the 'inverse square way'.

    Why make it clear that this is not an exotic phenomenon, but on the contrary quite 'the norm'?
    Because despite it being so obvious that the OP's hunch about the light fall off was spot on, it gave rise to some serious doubt, and hard work explaining that there is nothing strange going on.

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