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Thread: question on effects of ISO on exposure

  1. #1
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    Default question on effects of ISO on exposure

    Hey guys, i've got this question on using ISO to affect exposure.

    I understand that upping the ISO rating say from 100 to 400 would mean that the film is 4 times more sensitive to light.

    so in short, this allows me to set a shutter speed 2 stops faster say 1/250 to 1/1000 at a give F-stop value right?

    so flowing with this logic, if i up the rating of film from 100 to 1600, would it mean that the film is 16 times more sensitive to light and therefore would allow me to set a shutter speed 8 times faster at the same given F-stop value to give me the same exposure if i had originally stayed with the film rating of ISO 100?


    would appreciate any confirmation on this...thanks guys!

  2. #2

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    Yes, except that as you mentioned, since it's 16 times as sensitive, it should be 16 times faster, not 8. So a 1s exposure becomes 1/15s (I think there isn't 1/16s).

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    Quote Originally Posted by darkness
    Yes, except that as you mentioned, since it's 16 times as sensitive, it should be 16 times faster, not 8. So a 1s exposure becomes 1/15s (I think there isn't 1/16s).
    which means i set the shutter speed 16 times faster and not 8 times?given that i up my iso from 100-1600 shouldn't the equivalent be 8 times instead since an incremental of one stop would mean the doubling of light let in?

    was going through this thought process...so hope my doubts on this can be cleared up

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    100>200>400>800>1600
    Increase in exposure = 2x2x2x2 = 16 times more sensitive.
    But you get only 4 stops faster shutter speed.
    So if originally you were shooting at 1/30, 4 stops will be 1/60, 1/125, 1/250, 1/500.

    However, that is IF you can find a film that can be pushed 4 stops and still give reasonable results.

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    Since we are on this...

    anybody know if there is a mathematical relation btw Aperture & Shutter Speed?

    e.g. F4, 1/60s, F8 ???s

    Also, for "normal" 35mm film eg Kodak, Fuji etc. how is the push performance?

    100 -> ??? ISO

    What about the reverse?

    400 -> ??? ISO

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    every change of aperture by 1.4x, is a stop difference. so f8 is 2 stop smaller than f4 and the shutter speed will be 1/15s for the same scenery.

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    Quote Originally Posted by Castlesinthesky
    which means i set the shutter speed 16 times faster and not 8 times?given that i up my iso from 100-1600 shouldn't the equivalent be 8 times instead since an incremental of one stop would mean the doubling of light let in?

    was going through this thought process...so hope my doubts on this can be cleared up
    Just to add, when you push your film like you mentioned, the grainier your shots will become. Not to also mention that when you push process your film, its gonna cost more.

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    Quote Originally Posted by Prismatic
    100>200>400>800>1600
    Increase in exposure = 2x2x2x2 = 16 times more sensitive.
    But you get only 4 stops faster shutter speed.
    So if originally you were shooting at 1/30, 4 stops will be 1/60, 1/125, 1/250, 1/500.

    However, that is IF you can find a film that can be pushed 4 stops and still give reasonable results.
    how did you derive 4 stops faster looking at it?it's done by counting number of times it's doubled which in this case is 4 times right?

    so if i up the iso from 100-800, which would be 8 times more sensitive

    2x2x2=8 times more sensitive

    would me that it would give me 3 stops faster shutter speed right?

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    Quote Originally Posted by mpenza
    every change of aperture by 1.4x, is a stop difference. so f8 is 2 stop smaller than f4 and the shutter speed will be 1/15s for the same scenery.
    to be really acurate, its not 1.4 but rather root 2... (square root of 2)
    just for everyones info...

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    Quote Originally Posted by Castlesinthesky
    how did you derive 4 stops faster looking at it?it's done by counting number of times it's doubled which in this case is 4 times right?

    so if i up the iso from 100-800, which would be 8 times more sensitive

    2x2x2=8 times more sensitive

    would me that it would give me 3 stops faster shutter speed right?
    Correct. But most film are only good for pushing up to 2 stops. Anything more, you get grainy pictures, unnatural contrast and colours.

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