ASPECTS OF DIGITAL PHOTGRAPHY
CAPTURING THE IMAGE
In principle conventional and digital photography are similar. In both systems the effect of light falling on a sensitive ‘device’ is stored and later ‘processed’ to produce the final image in the form of a print or a picture projected on a screen. In both technologies the requirements for a good picture are the same, namely, a good eye and good timing (the critical moment).
In film, light ihfilling on the emulsion produces a change in the film, which is stored by it until processed (developed) to a negative or positive image. The film is then used to produce a print or, in the case of the positive image, a projected picture.
In the digital camera the device that is the equivalent of the film is usually a ‘Charge Coupled Device’ or CCD. The effect of light on this device is immediately measured and processed in the camera by an on board ‘chip’.
The result is stored by a special type of memory chip. The information on this chip is processed later in a computer coupled to a printer or video projector, to produce a print or projected picture.CHARGE COUPLED DEVICE or CCD
A CCD is usually chosen as the picture element in cameras aimed at the domestic and hobby market Generally these are cameras priced up to about £1500 (July 2002). Other devices ‘on the horizon’ will be discussed later.
In the broadest of terms a CCD is a rectangular array of photo diodes with associated electronics assembled on a silicon chip. There are 4 diodes horizontally for every 3 vertically which produces the usual 4 x 3 aspect ratio of the digital camera. In essence every diode contributes one Picture (Pix) Element (el) or Pixel to the finished picture. CCDs are available with pixel counts ranging from around 400,000 to over 6 million, the diagonal of the array ranging from 1/3-inch to 5/8-inch. The size of the individual pixel lies in the range from 12 micrometers to 3 micrometers.
When light thus on a pixel it developsan electric charge proportional to the intensity of the light. The charge on each pixel must be read and the accumulated data processed to produce information from which a picture may be printed. An outline of the method used is given below.
The charges on the last row of the CCD are moved to a position where their values can be measured analogue ihahion. These values are amplified, then converted to digital readings via an A-D (analogue to digital) converter. The values are then stored temporarily on a memory chip. All the charges on the chip are moved down one row and the output and measurement process repeated. It is this movement (coupling) of charges between rows that gives the device its name. In sequence all the charge information is moved to a digital store.
The photo diodes are colour blind, yet colour pictures are required. This is achieved by placing a colour filter over each pixel. The filters are Red, Green and Blue in colour (not necessarily the same number of each) and each pixel is often ‘topped’ by a micro lens to improve light gathering and so improve performance under poor light conditions.
The data stored in the temporary digital store has now to be processed to assign a true colour to each pixel. This is done by examining the brightness of the colours on it and the surrounding pixels and blending these values into a composite colour value for that pixel.
Values for the outer rows of pixels can’t be derived by this method and so the effective pixel count is always less than the number of pixels on the CCD. The creation of an image in a digital camera is essentially the result of a sampling process. It follows that the final resolution in the picture will be heavily dependent on the number of samples taken or, in other words, the number of pixels on the CCD. Experience to date suggests that the minimum specification for a digital camera is a 3x zoom lens (35 to110mm equivalent) and a CCD with a pixel count of 3M, if serious amateur photographic work is to be contemplated.