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Thread: How do adjustable ISOs work?

  1. #1
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    Default How do adjustable ISOs work?

    hi there fellow Csers, i would like to know more about the technicalities of ISOs on SLRs..

    how does the manual over-ride of ISO value work? if the ISO of each and every film is different, then how does changing the ISO value on the camera dial correspondingly affect the difference in grain that the final picture has?

    the higher the ISO value----->the larger the size of grains of silver halide ions ( or something like that ) on the light-sensitive film surface itself----->larger crystals are more sensitive to light---->the better it is for low light photography, but sacrificing smoother grain, i.e grain gets higher with ISO value

    read up on a very comprehensive book 'Photography' a month or two ago and learnt a few things. so my question is how the electronically configured ISO on the SLR can change the physical property of a fixed ISO value for the film itself.

    enlightenment, please!

  2. #2
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    Quote Originally Posted by doug3fflux
    so my question is how the electronically configured ISO on the SLR can change the physical property of a fixed ISO value for the film itself.
    It doesn't, because it can't. All the setting does is tell the light meter what film sensitivity it should assume in determining exposure.

  3. #3
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    It does not. It is how you process the re-rated film that changes the physical properties. If you do not compensate for the new iso, then the film will just come out under/over exposed.

    ISO value is not the only factor in grain control. Choice of developer and technique of agitation while development also matters.

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    oh.. i see i see. so if i adjust the ISO to 64 when i am using a 200 film, the light meter in the camera becomes more sensitive to light i.e it will purposely overexpose the film?

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    Quote Originally Posted by doug3fflux
    oh.. i see i see. so if i adjust the ISO to 64 when i am using a 200 film, the light meter in the camera becomes more sensitive to light i.e it will purposely overexpose the film?
    It won't change the sensitivity - it will "shift" the translation table between measured light intensity and recommended exposure. Choosing a higher sensitivity setting will lead to underexposed pictures, as the film is less sensitive than the camera thinks it is.

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