light
Aperture priority lets you set the aperture you want and the camera sets the shutter speed for you. You use it when you want to control depth of field. Shutter priority lets you set the shutter speed you want and the camera sets the aperture for you. You use it when you want to control how motion is rendered in the photograph. Program mode sets both shutter speed and aperture for you. Use it when you are not concerned with aperture or shutter speed control.
shutter speed and lens aperture
Aperture Priority has the camera set the shutter speed for you allowing you to set the aperture and it will set what shutter speed it thinks is best for your current light situation.
The amount of time that the shutter remains open - allowing light to pass through it to form the image. Generally - a lower shutter speed would be combined with a smaller aperture and a higher shutter speed with a larger aperture to correctly expose the image.
Aperture and shutter speed control the amount of light that passes from the lens to the film or digital sensor of a camera. Aperture is the size of the opening within the lens. The lower the f-stop number (1.4 for example) the larger the opening and the more light is passed through. Shutter speed is closely related. It is the amount of time that the lens is open. The combination of the size of the opening in the lens and the amount of time that the lens is open determine the exposure.
The shutter speed setting controls the picture exposure, or how dark or bright the picture will be. Wikipedia has some very good information on shutter speed and aperture.
Shutter speed, aperture, and ISO are the three main factors that control exposure in photography. Shutter speed determines how long the sensor is exposed to light, aperture controls the amount of light entering the camera, and ISO adjusts the camera's sensitivity to light.
Exposure, in digital or film photography, is determined by aperture and shutter speed. On a manual camera, the user selects both values.On an automatic camera, there are four possibilities:Manual mode. User selects both aperture and shutter speed.Shutter priority. User picks the shutter speed and the camera adjusts the aperture to ensure proper exposure.Aperture priority. User picks the aperture and the camera adjusts the shutter speed to ensure proper exposure.Program mode. The camera selects both values.
Well when the aperture is wide enough and shutter speed is not fast enough then there will be enough amount of light for the photo to be taken and looking good enough.
Usually, one would change the aperture to compensate for the change in shutter speed in order to keep the level of exposure the same. Changing the aperture affects the depth of field.
Exposure is the combination of aperture and shutter speed. Aperture (measured in f-stops) determines the amount of light that will enter your camera. Shutter speed (measured in seconds) determines the amount of time that the shutter will stay open to allow light to enter.
High shutter speed and fast lens/Big open aperture.