ISO Truly Applies to Film, But What About Digital?

Discussion in 'Digital Discussion & Q&A' started by grafiks, Apr 14, 2007.

  1. grafiks

    grafiks TPF Noob!

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    Sorry if this has been talked about before.

    Most people familiar with photography know what ISO means when talking about film. One thing I have never been able to understand is how ISO can relate to a sensor inside a digital camera. How can the sensor become more sensitive by changing the ISO? I can only guess that it is a software function, which to me is pretty lame. It makes me think that ISO is just a "feel good" setting on digital cameras.

    I personally do not use high ISO on digital. I simply slow the shutter speed instead. Of course, I have experimented with the setting and saw the extra grain with the high ISO settings.

    Is there anyone that can explain why an ISO setting is included with a digital camera and does it really work? And if so, how?

    Thanks!
     
  2. Alex_B

    Alex_B No longer a newbie, moving up!

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    well, the signal from each sensor pixel is amplified by the same amount. This amplification factor is arbitrary and hence is something which can be chosen by the photographer. In order to give you the same feeling the amplification scale was calibrated to match film ISO.

    To get the right exposure, you choose the right aperture, exposure time .... and ISO. So this ISO is just as real as film ISO is real, it just stems from different physics / chemistry in the two cases. Just see film as a chemical and disposable version of the sensor/amplifier combination.

    A smaller amplitude signal has to be amplified more, which means also the noise is amplified more (you have an overall worse signal to noise ratio with weaker signals). Hence higher ISO - more noise. Just like higher ISO for fim means more grain mostly.
     
  3. grafiks

    grafiks TPF Noob!

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    That's kind of how I thought it would have to work on a digital camera. So it would be software driven, telling the sensor to become more sensitive (in other words, to amplify more or less depending on the current ISO setting). It still leads me to believe that a longer shutter would yield a superior image by capturing a broader range of light with less graininess.
     
  4. zioneffect564

    zioneffect564 TPF Noob!

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    Yes i keep my camera at 100 iso for everything but there are times when you cant use low iso for example you're in a place with low light and you dont have a tripod. For me i would bump up the iso because you can fix noise but you cant fix camera shake.
     
  5. grafiks

    grafiks TPF Noob!

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    True. That would be the only good reason to use it, that I can think of.
     
  6. Digital Matt

    Digital Matt alter ego: Analog Matt

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    The same is true of film. Lower ISO films produce images with much tighter grain.
     
  7. darich

    darich TPF Noob!

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    But increased ISO also lets you freeze a moving object that your longer exposure can't so that's 2 reasons why adjustable ISO is useful.

    ISO is included for exactly the same reasons that you get various speeds of film.
     
  8. grafiks

    grafiks TPF Noob!

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    It does make sense to have it for the purpose of freezing fast objects in low light.
     
  9. Digital Matt

    Digital Matt alter ego: Analog Matt

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    Not just for freezing the subject, but also the motion of the photographer.
     
  10. Alex_B

    Alex_B No longer a newbie, moving up!

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    well, lets just agree on the fact that it has the same effect exposure wise as on film .. and if you take the analogy grain / noise, it even holds true for the image appearance ;) (of course grain looks slightly different from noise, noise can be slightly less pleasing)
     
  11. grafiks

    grafiks TPF Noob!

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    Thanks for all the input. It makes much better sense to me now (regarding how the ISO setting works and is used with digital).
     
  12. Don Simon

    Don Simon TPF Noob!

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    I think often it's not helpful to draw analogies between different technologies or systems but here it makes sense; sensitivity is just as integral a factor in exposure with digital as it is with film. As for using low ISOs where possible, yes ideally one would do that in order to keep noise to an absolute minimum. But it's not always practical to do that. I think most would agree that it's better to have a noisy sharp image than a clean but blurred one. Software can reduce noise quite well, and anyway I think most dSLRs now can shoot at least up to ISO 400 without noise being all that noticeable.
     

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