Why not negative ISO?

Discussion in 'Photography Beginners' Forum' started by dEARlEADER, Apr 22, 2008.

  1. dEARlEADER

    dEARlEADER TPF Noob!

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    Sorry to bother the forum with these kinds of questions but.. hey... this is the beginners place....

    If we adjust ISO upward to amplify the sensor... why can't deamplify the sensor for longer shutter opening times without blowing out highlights??(I mean deamplify past ISO100)

    This would eliminate the use of neutral density filters...

    I understand this is what exp comp is kinda for... but I also understand the exp comp adjustments just affect apt/shutr to create the desired effect.. this is useless in manual mode...

    please correct me if i am wrong..
     
  2. Helen B

    Helen B TPF Noob!

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    The problem is that the sensor itself gets saturated as you give it too much light, and as you 'de-amplify' the signal you cannot recover highlights that are over the sensor's saturation level.

    Using the sensor at an ISO below its native ISO helps with noise in the shadows, but it does reduce the headroom for the highlights - ie it makes the shadows better, but it may clip bright highlights. The native ISO of many modern sensors is around 160.

    ND filters are very useful in this respect, especially for video cameras for which using fast shutter speeds is not aesthetically acceptable in many cases, though it is technically possible.

    As an aside, the arithmetic ISO (ie the one that goes from 100 to 200 in one stop) can never be negative. It can get closer and closer to zero, but never below it.

    The rarely used logarithmic ISO speed (ie the one that goes from 21 to 24 in one stop) can go below zero.

    Best,
    Helen
     
  3. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    You are right...there is no 'exposure compensation' in manual mode...you simply adjust the settings away from the meter reading (or use another method of metering).

    As for negative ISO...I don't think so. The way I understand it...the lowest, or base ISO, is the signal from the sensor with no amplification...which gives you the cleanest image. If you need more sensitivity, you turn up the ISO which amplifies the signal...but this introduces noise into the images.

    So, to answer the question...I don't think they can go any lower than 'no amplification'.

    That being said, some cameras to have lower ISO settings...their base ISO is lower. I believe that some medium format cameras have this and even some DSLR models. This is especially useful for certain types of photography...studio photography comes to mind.

    So why don't more cameras have a lower base ISO? I guess it's because then they would have to have more amplification to get to ISO 800 or 1600...and it's probably more useful to have a clean ISO 1600 than to have ISO 25.

    I'm curious to see where the technology is going. The newest cameras are very good at ISO 800 and 1600...and some are topping out at ISO 12000 or there abouts.
     
  4. Socrates

    Socrates TPF Noob!

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    Never heard of that. Is it equivalent to DIN?
     
  5. dEARlEADER

    dEARlEADER TPF Noob!

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    Thanks Big Mike....

    and as for Helen B.... this woman has the CRAZY info... like a Pez dispenser of camera information....

    I wish I could fold you up and put you in my back pocket.....and unfold you on my shoots.....
     
  6. Alpha

    Alpha Troll Extraordinaire

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    Sigh...

    The high native ISO of 35mm sensors is probably the biggest reason I won't buy one. Without the use of ND filters, this can be really obnoxiously limiting when shooting in bright conditions (bright sunlight or high-powered strobes).
     
  7. Socrates

    Socrates TPF Noob!

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    :thumbup:

    I still have the futile hope that I can identify something, anything, that I know and she doesn't!
     
  8. Helen B

    Helen B TPF Noob!

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    Pretty much. Exact definitions have changed through the decades, but that's what became of DIN. When the ISOs for film speed came out, both were quoted, but the arithmetic one has rather overwhelmed the logarithmic one. They are both still defined in the relevant standards for film speed.

    ASA became ANSI. ANSI leads the ISO technical committee that do photography things, so it's not so much that ISO took over ASA, it's more like the other way round.

    Best,
    Helen
     
  9. Socrates

    Socrates TPF Noob!

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    Thanks.
     
  10. Mav

    Mav TPF Noob!

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    A device with negative ISO would be emitting light, not receiving it! :lol:
     
  11. Solthar

    Solthar TPF Noob!

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    ... a 1DS Mark III that doubles as a flashlight? Where do I sign up?
     

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