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ISO Has Nothing to Do with Exposure: A Technical Explanation


Photographer Chris Lee of the YouTube channel pal2tech recently released one of the most straightforward technical explanations of ISO that we’ve seen. In this 12 minute video, he explains what ISO is and, just as importantly, takes on a couple of common misconceptions by explaining what ISO is not.

ISO is probably one of the (if not THE) most misunderstood terms as it relates to digital photography. Stemming in part from people equating ISO sensitivity directly with film speed, and in part from some useful-but-misleading simplifications that are shared quite frequently, people often share two bits of misinformation:

  1. ISO is one way to increase your exposure without changing shutter speed or aperture
  2. ISO “increases your sensor’s sensitivity to light.”

As Lee explains in the video above, neither of these things is technically true, though both ARE useful ways to think about ISO when you’re out shooting.

ISO is a gain knob. Electrical amplification that is done after your camera is done gathering light. It has no impact on how much light your camera sensor’s photosites can gather during a given exposure, and therefore has no direct connection to exposure itself, despite being part of “the exposure triangle.”

At the most basic level—and Lee plans to do a follow-up explaining more in-depth concepts like ISO invariance and how different cameras handle this setting—ISO is the level of electrical amplification done to the analog “signal” collected by your image sensor before it’s sent to the analog to digital converters (ADCs), eventually producing an image.

Lee does a fantastic job of explaining all of this in easy-to-understand terms—with visual aids no less—in the video above, which we highly recommend you watch if you’re still confused about what ISO is and is not. And if you want to dive even deeper, keep an eye on his channel in the coming weeks for more great educational content about ISO.