In the first part of this video we learned what a stop is, and how to use shutter speed to adjust our exposure by an arbitrary number of stops. In the second part we learned how to use aperture and ISO to do the same.

We also learned that no matter which setting you’re changing, a stop is a stop is a stop. Which means that if you use your shutter speed to darken your photo by one stop you can then use your aperture to brighten your photo by one stop and those two photos will have the exact same exposure. However, they’ll have completely different settings.

And this is incredibly cool because it allows you to make creative decisions about your photography. Say you’re shooting a waterfall at ISO100, f/8, and 1/4 second. Now the water looks ok, but say you want it to become even silkier, well then you need to increase your shutter speed.

If you double that quarter second, and then double it again to get to one second, you’ve got a much longer shutter. But you’ve also increased your exposure by two stops. So in order to not blow out your photo, you’ve got to take away those two stops somewhere else, either in ISO or aperture.

And most likely you’re going to want to use your aperture. In order to darken things by two stops we need to go from f/8 to f/11 (one stop) to f/16 (two stops). And now you can see our exposure is exactly the same as it was before, but we’ve got a much longer shutter, and silkier water to show for it.

Let’s take a look at another example, of some flowers and mountains. If I shoot at 1/100 sec and f/8 it will put most of the scene in sharp focus. But say I wanted to use a shallow depth of field to draw more attention to the flowers. Well then I could open up my aperture to f/4, which gives me a nice shallow DOF, but also increases my exposure by two stops.

So then to compensate for that, I need to decrease my shutter speed by two stops, from 1/100 to 1/200 to 1/400. Again, I get the exact same exposure, but with a completely different creative result.

And of course you can throw ISO in there as well to give you one more variable to play with so you can get the exact aperture and shutter speed you want. No get out there and practice because when you really understand how to do this, that’s when photography becomes so much fun because it lets you inject your creative vision into your photos.

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