Digital Photography-I

Digital cameras use charge coupled devices (CCDs), instead of film, to

record the light levels in the image being captured. CCDs are electronic

chips that generate an electronic signal when light strikes one of its photosites.

The brighter the light, the stronger the signal. The signals from each

photosite are collected, digitised and stored. The stored value from each

photosite is one sample point in the capture of the image.

There are two ‘styles’ of CCD:

❑ the linear array

❑ the area array

The smaller the photosites, and the closer they are together, the finer

the detail that can be recorded. The resolution of the picture captured

relates directly to the number of photosites.

In the case of the linear array, the horizontal resolution is the number

of photosites, but the vertical resolution is fixed by the way the image is

moved across the sensor. The image must be ‘stepped’ across the sensor

so that lines of picture information are generated. The number of lines

(steps) form the vertical resolution. The whole picture resolution can be

calculated by multiplying the number of photosites by the number of steps

for the physical size of the sensor.

The area array, though, is used in much the same way as a piece of film:

the whole image is projected onto the sensor and captured in one ‘take’. In

the case of the area array, both the horizontal and the vertical count of photosites

are fixed. The total picture resolution can be calculated by multiplying

the horizontal by the vertical for the physical area of the device.

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