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An automatic exposure mode that sets the camera's aperture and shutter speed often referred to on camera dials and panels as P. What sets it apart from Auto mode, is that all the other functions, including exposure compensation, can still be adjusted.
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This is how the camera adjusts the shutter speed and aperture to ensure the right amount of light reaches the film or CCD. Early cameras only had a manual mode (M) where the user had to select the aperture and shutter speed manually to ensure the correct exposure. Over the years cameras have become more sophisticated and now offer several automated modes including Program (P) - a fully automatic exposure mode that sets the aperture and shutter speed; Aperture priority mode (AP) where the user selects the aperture and the camera sets the necessary shutter speed; and shutter priority (SP, or Tv on some cameras) where the user selects the shutter speed and the camera sets the necessary aperture. Auto bracketing (AB) takes a pre-selected number of photographs, one at the suggested exposure and one to either side, so you can be sure of one accurate result. There are also several subject-based program modes that we haven't listed here that tailor the camera for particular subjects such as sports (action), landscapes, portraits, or flowers (close-ups). Some digital cameras have black & white and sepia modes. Buying advice A full auto program mode is ideal for point-and-shoot photography, but it's also useful to have some control over the exposure. The beauty with digital is that you can see whether the camera has got the shot right by previewing the image on the LCD. If not, you try again. If there is no manual control you can often preset the exposure using an auto-exposure lock or exposure compensation. The subject based program modes are often a waste of time and don't really bring much to the package. Special effects modes on digital cameras are also throw-away because all these can be created using the computer later.
Having an interchangeable viewfinder increases your scope to shoot from low down using a waist-level finder or to enable program exposure using an AE head. There are also options with magnifying hoods. Many medium format cameras have this feature, but only a couple of 35mm SLRs do.