Most digital SLRs have a dial on the top that gives the photographer various options for how to adjust the image outcome. The (M) or manual setting allows the photographer to adjust shutter speed and aperture manually. There is usually a little meter that shows + or – indicating whether the shot will turn out over or underexposed, but the photographer can choose to take the picture regardless. For new photographers it is probably better to start with one of the easier to use semi-automatic modes such as shutter priority or aperture priority.
Shutter priority (usually denoted S or Tv) means the photographer sets their desired shutter speed and the camera sets the best aperture to get the correct exposure. This is a useful setting when the photographer is more concerned about the shutter speed than the aperture. For instance, if a photographer wants to get a slightly blurred picture of a runner, to indicate motion, they should set a relatively slow shutter speed. Or, to stop motion, the photographer should set a faster shutter speed.
Aperture priority (usually denoted A or Av) means that the photographer chooses the aperture and the camera sets the shutter speed to make a correct exposure. This is useful when the photographer knows their desired aperture, but doesn’t particularly care about the shutter speed. For instance, in food photography, getting a relatively shallow depth of field is often desireable, so the photographer might set the aperture to a relatively large diameter. Aperture priority and shutter priority are usually referred to as semi-automatic, and are typically easier to use than manual.
Digital SLRs usually allow the photographer to choose their desired ISO. Some DSLRs have an automatic mode where the camera will choose the ISO. This can be useful, especially on aperture priority, where the photographer might not notice that the shutter speed is getting too low to hand-hold. The camera will usually double the ISO, which allows a faster shutter speed for the same exposure.
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