1st Aperture/F-Stop, 2nd shutter speed and 3rd ISO/ASA
These 3 settings depend on one another and their interaction with light. A photograph is created by a chemical process where light is exposed to film, or a sensor in digital cameras. I always shoot on manual mode which gives me complete control on how I want to capture each image. I usually start my clients on Aperture Priority.
F/2.8 is wide open and allows the greatest amount of light to hit the Film or Sensor for your shortest possible exposure in a low light situation. F/2.8 will also give you the shallowest depth of field. When you have a shallow depth of field your main subject matter can stay in focus but your background will become a soft blur.
F/22 is stopped down and allows the least amount of light to hit the Film or Sensor. F/22 if used correctly will give you an infinite depth of field. Stopping down all the way requires a longer exposure which can cause motion blur
When you stop down two stops from F/2.8 to F/5.6 1/4th the amount of light will hit the Film or Sensor if your shutter speed and ISO/ASA remains the same.
You can either decrease your shutter speed by 400%
Or increase your ISO/ASA by 400%
F/2.8 at 1/120 sec with an ISO/ASA 100
F/5.6 at 1/30 sec with an ISO/ASA 100
F/5.6 at 1/120 sec with an ISO/ASA 400
If your exposure is too long you have a chance of motion blur
If your ISO/ASA is too high you will create noise