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Reference: P, S, A, and M Modes
Exposure
The exposure (brightness) of photographs is determined by the amount of light that falls on the image
sensor (CCD) while the shutter is open. Two factors that determine exposure are shutter speed and
aperture.
Aperture refers to the size of the opening through which the light passes. The larger the aperture, the
greater the amount of light that passes through the opening and the brighter the exposure. Smaller
apertures mean less light and darker exposures. The camera displays show aperture in “f/-numbers”:
the larger the f/-number, the smaller the aperture.
Shutter speed determines how long the shutter is open. The slower the shutter speed, the longer the
image sensor is exposed to light and the brighter the exposure. Faster shutter speeds mean that the
image sensor is exposed to light for less time, producing darker exposures.
The relationship between shutter speed and aperture can be likened to fi lling a cup from a tap. In
this analogy, the amount of water needed to fi ll the cup is the amount of light needed for optimal
exposure. If the cup overfl ows, the picture will be overexposed. If the cup doesn’t fi ll, the picture will
be underexposed. The amount the tap is opened is the aperture, and the length of time the tap is
opened the shutter speed. Open up the tap and the cup will fi ll in less time; tighten the tap and more
time will be needed to fi ll the cup.
Just as a cup can be fi lled in diff erent times using diff erent tap settings, diff erent combinations of
shutter speed and aperture can be used to produce the same exposure. The results, however, will be
very diff erent: fast shutter speeds and larger apertures freeze moving objects and soften background
details, while slow shutter speeds and small apertures blur moving objects and bring out background
details.
Fast shutter speed (/ s)
Slow shutter speed (1 s)
Small aperture (f/36)
Large aperture (f/3)
(Remember, the larger the f/-number, the
smaller the aperture.)
