Digital rather than film• Rather than using a film, the digital still camera contains a sensor which converts light into electrical charges.• The sensor is either a charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) technology.• The sensor changes the light to electrons, which then means it can read the accumulated charge of each cell in the image.
CCDs• A CCD moves the charge across the chip so it can read it a corner of the array.• Something called an analogue-to-digital converter (ADC) turns each pixel’s value into a digital value• It does this by measuring the amount of charge at each photosite and converting it into binary form• The advantage of CCDs mean that high-quality images are produced. However, they consume 100 times as much power as a CMOS sensor
CMOS• A CMOS device uses transistors at each pixel to amplify and move the charge using wires.• The advantage of a CMOS sensor is that it consumes less power although they are much more susceptible to noise.
Capturing Colour• As each photosite is colourblind, the sensors have to use filters to look at the light in the primary colours of light. This allows the camera to create a full spectrum.
Capturing Colour – Beam Splitter• One way of recording the three colours in a DSC is to use a beam splitter.• This directs the light to different sensors• Each sensor sees the same image, but due to the filter, each sensor only responds to its appropriate colour.• The advantage of this is that the camera can record each colour at each pixel location
Capturing Colour - Rotation• Another method is to rotate red, blue and green filters in front of one sensor• The sensor records three different images in succession.• This allows the information of the three colours to be recorded at each pixel location• Unfortunately, as the images aren’t taken at the same time, the camera and target must stay stationary for the three readings.
Capturing Colour - Filtering• A cheaper method to record the primary colours is by placing a permanent filter called a ‘colour filter array’ over each photosite.• This means information around each sensor can guess what the true colour is at the location. This is called ‘interpolation’• The most common filter pattern is the Bayer filter pattern.• The advantage of this is that only one sensor is needed, and that all colours are recorded in the same moments.
Taking a Photo – CCD• After lightly pressing the shutter release, the camera focuses on the subject and takes a reading of the existing light• The camera sets the aperture and shutter speed for the best exposure• After pressing the shutter release down all the way, the camera resets the CCD and exposes it to the light, which builds up an electrical charge until the shutter closes.• The ADC measures the charge and creates a digital signal• The process interpolates the data to cerate natural colour. You can now see the output on the LCD screen.
Storing the Photos• To make the most of the storage space in removable storage devices (SD Cards) used in digital cameras, is by using a form of data compression which makes the files smaller.• Two features of digital images make this possible; repetition and irrelevancy.• In a photo, patterns develop in the colours. The compression takes advantage of these repeating patterns so that there is no loss of information, Unfortunately, only up to 50% is reduced.• When the camera takes the picture, more information is recorded than the human can detect. This means that this information can be thrown away during the compression routine.