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  1. #1

    ISO and F stop!!

    These two things are confusing the heck out of me. I've read alot of stuff about them, but I'm still confused about how to use these settings. Can anyone please explain it to me!!!!! I would REALLY appreciate it!!

  2. #2
    Join Date
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    ISO is, basically, your last-port-of-call measure to get lighter pictures. Raising the ISO setting increases the sensitivity of the image sensor, meaning the sensor needs less light to expose the image. However the heightened sensitivity means the sensor picks up electrical signals in the sensor itself, which manifest as 'Noise', the nasty grainy stuff you really don't want on your images. So, if you have no other option - The ISO goes up to let you get the shot.

    F-stop refers to how wide the opening in your lens (Aperture) is - directly affecting the amount of light that gets to the sensor. A lower number, confusingly, means the aperture is wider. So, the lower the f/number is, the more light gets to your sensor. So, you can use a lower ISO, or a faster shutter speed. All good and well. However, the aperture also controls depth of field - A small aperture number (Such as f/2.8) will ensure that a very small portion of your photograph is in focus, and the background is pleasently blurred. A high f/number (say f/22) will make the area that is in focus much larger - Hence, a low f/number is desired for macro work, and a high f/number for landscapes. However, a low f/number is needed in low light to get you enough light without having to raise the ISO - It's a vicious circle really, and why low-light is a nightmare to photograph in.

    Don't know if it helped clarify things or just confused you more - But I tried.
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  3. #3
    Join Date
    Apr 2006
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    (I see Paradox has already replied while I was typing this lot in, but I'll post in anyway, because I hope it might add more depth to his concise reply).

    These are two things which, along with shutter speed, control the exposure of a photo. Too little exposure, your pic will be too dark. Too much, and it will be washed out.

    The f/stop, sometimes called the aperture, is a numbeer that describes the size of the hole in the lens that lets the light through, in a consistent way. To make it consistent between different lenses, it has to take the focal length in to account, and it's actually defined as the focal length divided by the optical size of the hole, so the confusing quirk is that for more light to get through, you need a smaller f/stop number, which means a bigger hole (aperture).

    All you really need to remember is a sequence of numbers, which go (from most light let through, to least)

    1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32, 45, 64

    Each of those f/stops or f/numbers lets half as much light through as the previous one (you're unlikely to come across any lens which has that full range of settings). This halving or doubling of light is known as a "stop", and the term is often applied to shutter spedds and ISO as well as aperture. Many cameras allow you to set intermediate values, which allow a bit more or less than half or twice as much light through, e.g. f/5.0 is about a third of a stop more light let through than f/5.6. These particular numbers are used because each f/number is roughly the square root of two times the previous one, and this compensates for the number being calculated from the diameter of the hole, but the amount of light let through depends on the area.

    A side effect of changing aperture is that it affects depth of field - if you use a smaller aperture (bigger f/number), more of the picture will be in sharp focus.

    ISO is a measure of hoe sensitive the camera sensor is to light. This commonly goes in the sequence (from least sensitive - needs most light to make a correct exposure - to most sensitive - needs least light):

    100, 200, 400, 800, 1600, 3200, 6400

    again with a halving of the light required at each step.

    In practice, you usually want to keep the ISO as low as possible, because generally, the higher the ISO, the noisier (grainy speccles of the wrong colour) your picture will be.

    So putting it all together, if on a cloudy day, a shutter speed of 1/100th of a second at f/5.6 on an ISO of 100 gave the correct exposure, but you want to use a faster exposure to freeze a fast moving subject, you might want to set 1/400 s and f/2.8 and keep the ISO at 100, or if you needed the depth of field of f/5.6 to get it all in sharp focus as well, you might want to increase the ISO to 400.

  4. #4
    Join Date
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    Understanding Exposure - Bryan Peterson.

    buy it and you'll never ask the question again.
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  5. #5
    Join Date
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    Quote Originally Posted by Rooz View Post
    Understanding Exposure - Bryan Peterson.

    buy it and you'll never ask the question again.
    Agreed! Great book

  6. #6
    Thanks!! I can finally understand it. It will just take me some time to get the hang of using all of it together.

  7. #7
    Join Date
    Apr 2006
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    242

    Play with this Flash animation

    http://dryreading.com/camera/index.html

    ....for how the variables of ISO/Shutter speed/Aperture/EV compensation will affect the exposure
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