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Thread: What really is ISO?

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    What really is ISO?

    Can someone explain what the various ISO ratings of a digital camera really mean? I hear quite often that you can create say ISO 3200 by shooting with a -2 exposure compensation setting. Is it really the same thing? Just looking for some good education! Thanks in advance to all contributors!

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    Deceased, but remembered fondly here... johnastovall's Avatar
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    Re: What really is ISO?


    "The market wants a Leica to be a Leica: the inheritor of tradition, the subject of lore, and indisputably a mark of status to own."
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    Senior Member M5-Guy's Avatar
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    Re: What really is ISO?

    Quote Originally Posted by bbodine9 View Post
    Can someone explain what the various ISO ratings of a digital camera really mean? I hear quite often that you can create say ISO 3200 by shooting with a -2 exposure compensation setting. Is it really the same thing? Just looking for some good education! Thanks in advance to all contributors!
    The last link was the geeky tech stuff, and does explain ISO..
    for the rest of us:

    It is a standard that allows us to go understand relationships between different elements of photographic controls.

    ISO (ASA in old school terms), is a rating that lets us know how sensitive film/sensor is to light. Higher number are more sensitive and require less exposure, the trade off is more grain/noise.

    It is the same ratio as f/stops and Shutter speeds.
    ISO 100/200/400/800 In 1 stop increments (because 1 stop is the accepted term... it really is 1/stop in one direction, or 1/2 stop in the other direction)
    ISO 100/125/160/200/250/320/400/500/640/800 in 1/3 stop increments

    f/stops f2 f/2.8 f/4 f/5.6 f/8 in 1 stop increments
    f/stops f/2.8 f/3.2 f/3.5 f/4.5 f/5 f/5.6 in 1/3 stop increments

    Shutter Speeds 1 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 in 1 stop increments

    Shutter Speeds 1/20 1/25 1/30 1/40 1/50 1/60 1/80 1/100 in 1/3 stops increments

    Now, each progression wither in full stops or 1/3 stops (going higher) will either 1/2 the exposure or 1/3 the exposure from the former setting. Wither it is ISO, f/stops or Shutter speeds

    NOW, Just adding to the EV setting to a WILL technically give you a higher ISO, BUT, It will be underexposed without in camera correction (Push Processing the JPG/RAW file), So, unless you shoot RAW, you probably won't have a usable file. and, even then, 2 stops underexposure will give a very dark JPG, and RAW. But, you may (May) be able to recover the RAW to a normal file.. but, you may also loose a lot of the shadows, since, they (the shadows) were exposed 2 stops under also.

    With Film... you have to change (Increase) the developer time to compensate for underexposure (the phrase "expose for the highlights, develop for the shadows") means, you slightly under expose to hold Highlight detail, and over develop to bring up the shadows. Same holds true for digital film.

    Cameras that have a "extended" range ISO (3200 or 6400) add an in camera "Push Processing" algorithm to help retain the shadow. Shoot RAW in any case.

    Hope This helps a little

  4. #4
    smithdsouza
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    Re: What really is ISO?

    iSO refers to film speed in digital cameras and it measures the photographic film's sensitivity to light. It determines how sensitive is image sensor to light. Insensitive films are termed as slow films and highly sensitive films are termed as fast films.

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    Subscriber Member Jorgen Udvang's Avatar
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    Re: What really is ISO?

    It's rather funny actually; formally, ISO doesn't have a meaning other than being the abbreviated name of the organisation that develops international standards, International Organization for Standardization. And don't ask me why it isn't called IOS.

    But it kind of follows a tradition, since DIN and ASA, which were previously used for film speed, are also names of organisations (Deutsches Institut für Normung and American Standards Association). Fortunately, in other areas, different measurement names are available (meter, Kw, joule, A0-4, 5, 6... etc.), even though many of them are ISO standards as well.

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    aprillove20
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    Re: What really is ISO?

    How does ISO used for filming?

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