At the heart of every digital camera is a light-sensitive
silicon chip called a sensor. The sensor takes the place of film
for recording images.  The size of these rectangular chips
varies depending on the type of camera. In small compact cameras
the sensors are relatively tiny, sometimes no larger than the
size of the fingernail on someone's little finger.  sensor size
explained  Above: The full-frame sensor in the Nikon D700  The
imperial measurements used to indicate some sensor's sizes are a
hangover from the old days of television camera tubes. They give
diagonal measurements as fractions of an inch and many
manufacturers do not publish exact dimensions for the sensors in
their compact cameras.  It can be very confusing to understand
and is best explained by a diagram showing the various relative
sizes of different types of sensors.  sensor sizes explained
Above: The relative size of sensors from the smallest type used
in a compact camera all the way up to a full-frame digital SLR
Sensors are packed w