Here is a cheat sheet I made up for all of my friends that are still having trouble understanding what makes for perfect exposure in your photos. These three elements are what makes a photo. They work together to make your photo exposed correctly or incorrectly depending on which mode you set your camera. Even when you are in AUTO mode, the camera “reads” the light and sets these 3 elements to the settings that the camera sees as the best exposure for the light that it “sees”. When you are shooting in Aperture Priority Mode, the camera will adjust the shutter speed based on the conditions that the camera interprets in order to expose your photo correctly. It will also adjust your ISO IF you have your camera set to AUTO ISO, Otherwise it will adjust the shutter speed based on the ISO that you have set. The same is true for Shutter Priority Mode, but reverse. Your camera will adjust the aperture based on the shutter speed you have set. The ONLY setting in which you have FULL control of the exposure is in Manual mode, where you set all three; ISO, Aperture and Shutter Speed.
Many of you have asked me what the “P” mode is. It is Program mode and it is similar to Auto mode as the camera sets the aperture AND shutter for you, but “P” mode allows you to set your ISO and whether or not to use the on camera flash. I hope this helps to demystify some of your camera’s settings. Feel free to download a copy of the cheat sheet here to carry in your camera bag!