Think about "Av" (aperture priority) as being the opposite of "Tv" (shutter priority) of each other (and vice-versa) i.e.:
Av = YOU set aperture first (==> it's the priority), then the camera controls the shutter speed based on its measurement of light.
Tv = YOU set the sutter speed first (again, ==> it's the priority), then the camera controls the aperture/opening based on light.
I guess earlier, you were in Tv mode, so even though you didn't realize it, you set the shutter speed and then the camera just chose the aperture for you based on light.
You'd want to pick one of the modes (Av or Tv) based on what you're trying to do.
For instance, if you want to take a pic with a lot of bokeh in the background, you will want the lens wide open and to stay wide open, so you'd use Av mode and set the lens wide open, then let the camera control the shutter speed.
On the other had, if you were shooting something with a lot of movement , you'd want the shutter speed to be fast, to freeze the motion, so you'd set the mode to Tv and set a high shutter speed, then let the camera control the aperture.
Or something like that
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