The Fundamentals of Exposure
Now you know that there are three fundamental settings that control how bright a scene is rendered by your camera: shutter speed, aperture, and sensitivity. Before we take a closer look at each, let's explore the different modes that digital cameras offer for changing these settings.
When you get your first DSLR or advanced compact camera, we recommend that you set it to Green, Auto, or P exposure mode on the mode dial (different cameras have different ways to set a fully automatic mode; consult your manual if your camera doesn't have a mode dial as shown here). The camera will then adjust itself to achieve correct exposure no matter the brightness of the scene.
When you have become familiar with the operation of your camera and are ready to take control over exposure, you should switch to the manual exposure control mode (M) until you master the basics of exposure. Then, going forward, you may find that the convenience offered by the semi-automatic modes on your camera is the way to go: Av (aperture priority), Tv (shutter priority), TAv (shutter and aperture priority), Sv (sensitivity priority), or P (program or hyper program). One of these will probably become your favorite mode. The semi-automatic modes still let you be in control of a specific aspect of the exposure while allowing the camera to do the rest of the work. For example, in aperture priority mode, you set the aperture and the camera will set the shutter speed.
We recommend that you start practicing exposure with a modern lens. Older lenses may impose restrictions on the shooting modes available to you. More details can be found at the end of this article.
Note that Pentax DSLRs with dual control wheels, when set to P mode, allow you to instantly override the shutter speed with one of the control wheels and the aperture with the other. This is called Hyper Program and is a Pentax-specific feature. Essentially, the result is that you can access Av or Tv mode without turning the mode dial.
Also of note is the green button found on all Pentax DSLRs. Pressing this button will reset your exposure customizations by reverting to the shutter speed and/or aperture values that the camera recommends.