Here is the first part of my post here is the link in case you missed it: Shutter Priority.
When the camera is set to Aperture Priority ( Av, A) you control the aperture and the camera will automaticaly select the Shutter Speed for proper exposure.
Aperture priority works in the same manner but you will have to watch your shutter speed (remember the guidelines)
The suggested safe shutter speed for hand held shooting is 1/60 at 50mm focal length when you change lens focal length the general suggested guideline is to simply place a 1 over the focal length.
Example: 250mm lens – safe Handheld minimum shutter speed is 1/250
Q: What does changing the Aperture do? More about Aperture AKA f-Stop
A: It does two things
1: Lets more light through the lens f1.4 or less Light f22
2: It gives the image more or less depth Depth Of Field which is how much of the scene will be in focus.
Examples – 50mm prime lens at f1.4 a very small area will be in focus and the rest will be blurred. At f22 with the focus set to 3-meters the image will be in focus from front to back.
Q: When would I use say f1.4?
A: Portraits of people, animals or any other objects you want to be the main focus in your image. You would also opt for f1.4 in low lighting conditions as this will let the maximum light through the lens.
Q: When would I use f13,f16 or f22?
A: For environmental portraits, landscapes, seascapes or any other environment that you want most if not all of the scene in focus.
Another helpful article The Exposure Triangle
Feel free to leave comments and questions.