Graphics Reference
In-Depth Information
worthwhile. You just need to ensure that the photographs are at least
reasonably well framed, sharp, and correctly exposed.
Exposure
Achieving a correctly exposed image involves a combination of separate
elements that require different settings on the camera. Exposure refers to the
amount of light falling onto a light-sensitive material or surface. Exposure
is determined by the shutter speed and the aperture setting. The correct
exposure of any given subject will vary under different light conditions and
is controlled by the photographer, who determines exactly how much light
should fall on the light-sensitive material, either film or the CCD in a digital
camera. Too much light and the image will be overexposed and too light;
too little light and the image will be underexposed and too dark. This is not
necessarily determined by either dark or bright conditions; the exposure can
be adjusted to account for that. The exposure is controlled by the amount and
duration of light falling onto the sensor and may be varied by changing the
shutter speed (the duration of light) or the aperture (the intensity of light). It
is often difficult to achieve a correct exposure for all elements in an image, so
some aspects may be under- or overexposed while the main element of the
photograph is correctly exposed.
(a)
(c)
(b)
FIG 7.21 a: Underexposed.
b: Overexposed. c: Correct exposure.
Lens Aperture
We have seen how the intensity of light is one element that determines the
exposure of an image. This intensity is controlled by the size of the aperture
that allows light to fall on the sensor. Rather like the eye's iris, the mechanism
in the camera, also called the iris , may be adjusted to allow more or less light
onto the sensor. The aperture may be opened or closed through a series of
settings across a range of diameters. These diameters are measured in units
known as f-stops. The lowest f-stop numbers denote the widest aperture
settings. The widest aperture setting, allowing the most light in, is f2.8; the
smallest aperture setting, allowing the least light in, is f22. A range of f-stops is
available between these two extreme settings, and cameras vary in how these
are adjusted. Some allow for third- or half-stop increments. In this way very
fine adjustments can be made.
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