Exposure and the sensor

Without getting into too much technical detail, I think a general understanding of our sensor and how our cameras capture light is useful.

With a little help from Michael Freeman's book on exposure, Perfect Exposure, this is an outline of the sensor and what it does.

D90 Camera sensor

The basic unit of the sensor is the photosite.

Each photosite collects the light for one pixel. Most DSLRs have a minimum of 10 millions pixels.  My camera, a Nikon D90 has 12 million.  The new D800 has 36 million.

The density of photosites on the sensor is measured as pixel pitch - the distance between the centre of one pixel to its neighbour.  The higher the pitch the better the resolution.

When light hits the sensor it is stored as an electrical charge in each photosite.  One photon excites one electron which is read as voltage.

The analogue to digital (ADC) converter converts the voltage into digital data.  This is monochrome information.  To get colour, a mosaic filter is fitted to the front of the sensor with the RGB pattern.

Mosaic filter

Demosaicing - is when the camera's processor has to interpolate the lost colour from the ACD procedure.

The digital values for each pixel across the area of the sensor can then be displayed as an image.

An average PC screen can display 256 distinct tones from black to white.  This gives us our brightness scale with 0 being black and 255 being pure white.

The camera also processes the image as we have seen in the first part of this course correcting errors and producing a pleasant image.