3 facts: ISO = light sensitivity - the amount of light needed to create an image. Shutter speed = the amount of light that the camera lets in to satisfy the sensitivity of the sensor at a given aperture (the amount of time needed to deliver the right amount of light). Aperture = the size of the hole that allows light to reach the sensor. Shutter speed and aperture work together to give the right amount of light that the sensor requires to produce an image. Working with AV (Aperture Value) allows you to dictate the the amount of pin-sharp or soft focus (depth of field) you require for your shot from a particular lens. Working with TV (Time Value) allows you to choose how fast or slow you want the reaction of incoming light to be to freeze the action. So, if you want to capture moving objects, consider working with TV. If you want to control the DOF of a lens work with AV. If you need a fast response under certain conditions - say street photography - try using Auto Settings, so you can concentrate on the creative moment. Once you get to understand how to use these settings, you may then want to give full manual a try. Good shooting and good luck!