P, S, A, and M
modes are referred to as "exposure modes" because they let you control settings that
determine exposure, namely shutter speed and aperture. Each of these modes offer different
degrees of control over shutter speed and aperture.
Exposure Mode: Mode P (Programmed Auto)
In mode P, the camera automatically adjusts shutter speed and aperture for optimal exposure. You can, however, choose other aperture and shutter speed combinations that will produce the same exposure: this is called "flexible program". Choose programmed auto for snapshots and in other situations in which you don't want to miss a shot.
Exposure Mode: Mode S (Shutter-Priority Auto)
You choose the shutter speed and let the camera automatically adjust aperture for optimal exposure. Use to emphasize motion.
"Shutter speed" is the time the shutter is open during an exposure. It is usually expressed in seconds or fractions of a second: 1 s, 1/2 s, 1/4 s ... 1/250 s, 1/500 s. Fast shutter speeds reduce the amount of light that reaches the image sensor, while slow shutter speeds increase it.
Exposure Mode: Mode A (Aperture-Priority Auto)
You choose the aperture and let the camera automatically adjust shutter speed for optimal exposure. Use to control background blur.
Aperture controls the amount of light reaching the image sensor and is usually expressed
as an "f-number": f/1.4, f/2, f/3.5, f/5.6, f/8, f/11, f/16, f/22, f/32.
Changing the f-number changes the size of the opening through which light enters the camera (the aperture) and thereby changes the amount of light passing through the lens. High f-numbers stop the aperture down, allowing less light into the camera, while low f-numbers increase the size of the aperture and let more light into the camera. Going from an aperture of f/4 to f/5.6 reduces the amount of light entering the camera by half and halves the brightness of the image on the image sensor.
Exposure Mode: Mode M (Manual)
You choose both the aperture and the shutter speed. Manual mode even offers a shutter speed of "bulb" for long exposures. Because you control both aperture and shutter speed, manual mode offers great scope for expression. But choose the wrong combination and your photo will be too bright or too dark, or in other words over- or under-exposed. Keep your eye on the exposure indicator when choosing aperture and shutter speed.