Just a quick fun photograph. I shot this image at an aperture of F22 in order to get the exit sign and the clock somewhat in focus. I was focusing on the switch which was pretty close to me.
Oftentimes you want the subject in sharp focus and the background blurred out. When that is what you want, choose a lower aperture number like F2.8 or F4. When you want more of your image in focus, choose a higher aperture number like I did here.