F stops represent a 100% change in light. So 1 stop is either 1/2 the light when closing or 100% when opening. Same shutter speed, one stop closed equals 1/2 the light hitting the sensor. One stop more open doubles the light hitting the sensor. So when working the algerbra to ISO, when you want to move the Fstop one down (closed) then you have to double your ISO to get the same exposure. ISO 200 would then have to go to 400, then 800, 1600, etc.
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