You can also use shutterspeed and iso to affect exposure. In this case it's a bit simpler: every time you DOUBLE or HALVE your number, youre moving a stop.
so,
from 100iso to 200iso is 1 stop.
from 100iso to 400iso is 2 stops (100 -> 200, 200 -> 400).
The same goes for shutterspeed.
1/60s to 1/125s is 1 stop
1/250s to 1/60s is 2 stops (1/250s -> 1/125s, 1/125s -> 1/60s)
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