Image Sensors

Digital camera image sensors consist of light sensitive semiconductors made of silicon diode. When light hits, the individual photodiodes (pixel) convert light into electrical voltage. These electrical signals are selected and transformed into brightness values.

We mainly distinguish between two different types of sensors- the CCD chip (Charge-Coupled Device) and the CMOS chip. The CMOS technology has the advantage of being cheaper to produce than the CCD technology and also uses less energy. The disadvantage is that it is more sensitive to image noise. However, this drawback can be somewhat rectified with the right technical skills.

Sensor – Beispiel CMOS-Sensor Canon [Foto: Canon]

Sensor – Example CMOS-Sensors Canon [Photo: Canon]

The silicon diodes of a CCD or a CMOS sensor are generally colour blind and only recognise a difference in brightness. To enable individual photo diodes to deliver colour information, a colour filter array (Bayer filter) is placed over the pixel sensor. A specific colour filter (red, green or blue) is placed over each pixel. Using this filter mosaic, the camera's internal image processor can process the values (with the aid of a software program - part of the firmware) and with the surrounding pixels, colour and brightness can be determined for each individual pixel. Additionally, the camera's internal software can determine white balance, contrast and colour saturation and can adjust it before the image is stored. Zudem errechnet die kamerainterne Software u. a. den Weißabgleich, Kontrast und Farbsättigung anpassen, bevor das Bildergebnis auf die Speicherkarte gelangt. As the human eye is very sensitive to green, for 4 pixels - a red, a blue and two green filters are placed over the pixel sensor. This results in a more accurate display of colour.

For the light sensitivity of image sensors the size of the silicon diodes is relevant. Their size usually relates directly to size of sensor used and the pixels detected. The determining factor here is usually the different camera sizes (medium format, APS-C, Four Thirds, CX format, compact cameras etc.).

In order to able to compare the focal lengths of various cameras, besides the focal length itself, you will need to know the size of the sensor i.e. the crop factor. A more detailed description about the crop factor and its effect on focal lengths can be found in the article "Crop factor".

Further information about the different types of image sensors: