getting used to thinking in Stops of light....
First we need to wrap our heads firmly around F stops as in aperture
Because that is how we measure the amount of light and set our camera to record the scene properly exposed.
Aperture in full nice round whole stops.....
f16 f11 f8 f5.6 f4.0 f 2.8 yes there are stops in between (halfs and even thirds and fourth)
Now here comes the curve ball...
if we choose a shutter speed of 1/120 second and we cut that in half
to a new setting of 1/60 shutter speed (making it half as fast) as in a
factor of 2
we have added One stop of light to our exposure
Because the shutter is staying open longer
If we double it (that shutter speed) from 1/120 to 1/240
then we have decreased the exposure by One Stop
Because the shutter is now open a shorter amount of time which equals less light into the camera
We can also adjust the sensitivity of the camera to light by changing
the ISO and if we double it we gain one stop, if we cut the ISO in
half we loose One full stop......example
we are at ISO 200 and we
decided we need to be able to shoot with two stops faster shutter speed
and we are currently at 1/30 of a second and an aperture of 5.6
from 1/30 to 1/60 is one stop, from 1/60 to 1/120 is our second stop we
want to gain in shutter speed, so the ISO needs to go to.....
800 ISO remember we started at ISO 200
double 200 to get 400 (a change of One stop) double it again to ISO 800
and we are at the second stop we needed to have the same exposure with a faster shutter speed
Here is an real world example....
If we are shooting at 1/60 of a second at an ISO of 200
at f4.0 and we are using a variable aperture lens and we need to zoom
towards the subject the lens just decreased the aperture (the nature of a
variable aperture lens) to lets say f5.6
Then we lost one stop of light and therefore must either slow down our shutter speed to 1/30 OR change our ISO to 400
To have the same exposure.