Thread: Aperture & ISO

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    Aperture & ISO
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    Ok let see if I get this correct, the Aperture determines how much light is coming into the camera and then the ISO determines how sensitive the camera is to that light coming into camera. I'm just trying to figure out how they affect each other or the overall image quality of the camera.


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    Moderator Alex H.'s Avatar
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    With actual film, which was made of layers photoreactive particles suspended on a strip of celluloid, ISO had to do with the light sensitivity of the particles. The higher the number, the more sensitive the particles, and the less light was needed for exposure. The tradeoff with high ISO film was noticeable grain. Above ISO 800 grain started to become quite visible, but sports photographers often relied on 1600 and 3200 film in order to capture action at high shutter speeds in lower light (stadiums, arenas, places where a flash wasn't of any help at all). Portraiture and lanscapes, subjects that required very fine detail and thus very fine grain, relied on slower ISO films like 64 and 100.

    In terms of DSLR cameras, ISO is really just a carryover term from film. What is really is, is digital gain. It doesn't as much adjust the sensitivity of the image sensor as it does amplify or attenuate the signal as the sensor becomes electrically charged. Again, there's a trade-off for using faster ISO speeds, but instead of film grain you get digital noise. Different chips have different "native" ISO ratings; that is, the ISO at which the signal is neither attenuated (reduced to a lower ISO) or amplified (raised to a higher ISO), so noise levels at different ISOs are not consistent from camera to camera. Basically, you want to keep your ISO low if at all possible (though staying at the native ISO of your camera is a good way to play it safe).

    Aperture and ISO do not interplay the way aperture and shutter speed do. When determining your desired exposure, choose aperture for the desired DoF and choose shutter speed for the desired motion capture. If those result in overexposure, add ND filtering. If they're underexposed, add light. If you don't have ND or additional light at your disposal, then bumping up the ISO is a last resort.
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    Quote Originally Posted by Alex H. View Post
    What is really is, is digital gain. It doesn't as much adjust the sensitivity of the image sensor as it does amplify or attenuate the signal as the sensor becomes electrically charged. Again, there's a trade-off for using faster ISO speeds, but instead of film grain you get digital noise. Different chips have different "native" ISO ratings; that is, the ISO at which the signal is neither attenuated (reduced to a lower ISO) or amplified (raised to a higher ISO), so noise levels at different ISOs are not consistent from camera to camera..
    Just a little correction, my friend:
    A digital camera sensor is like a fixed film, for example - ISO 100. This is camera's native ISO.
    When you toggle the ISO in log base 2 scale (e.g. 200, 400, 800, 1600 ....) you are actually using the analog amplifier before the A/D converter. It is called analog gain and it is the most superior way to gain up the footage.
    Any value between the ISO levels above will gain first to the closest analog ISO and then compensate with digital ISO (at 14bit). ISO 125 = ISO 100 + digital 25; ISO 160 = ISO 200 - digital 40. When boosting using digital gain, a lot of noise may appear (the video will be a lot noisier than higher analog ISO). Lowering using digital gain may reduce some of the noise of the dark areas but also may theoretically tighten the dynamic range.
    For instance: ISO 200 is better in any aspect than 125 (Canon DSLR).

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    Mark.

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