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The parts of the image which are sharply focused at the sensor surface
compared with the appearance of the objects which are either in front
or
behind of the principal point of focus give rise to this “out of focus” look. It
is determined by two elements, the lens focal length and its aperture.
You will find more of this important aspect later in the book.
The actual sensor itself has a fixed sensitivity to the photons which strike
the photosites. The bigger the photosite area the more photons can
release the electrons producing a bigger output voltage from the cell.
This voltage is generated from a very low output when no photons (no
light) strike the sensor to a maximum voltage when the photosite
becomes fully charged, or saturated.
This very small, analogue voltage contains an element of noise due to a
few factors, temperature – the higher the ambient temperature the higher
the noise, “shot noise” from random electrons being generated without
being triggered by a corresponding photon strike and from electrical
connection noise where the interconnections are made to the silicon
wafer.
This analogue voltage needs to be digitised so that the camera processor
can do the maths to calculate exposure.
This small analogue voltage is amplified by an electronic circuit and its
gain is directly related to the ISO value which we have come to
understand to be the measure of sensitivity of the camera.
In numerous publications there is the erroneous statement that the ISO
changes the sensor sensitivity to light. The sensor always has the same
output irrespective of the ISO set.
It is the amplifier which is connected to the output of the sensor
photosites which has the gain changed by the ISO setting. At the lower
ISO setting of 100 there is little, or no, gain applied to the photosite
output so both the signal and noise are amplified to the same amount.
As we increase the camera ISO setting we change the amplifier gain to
make the output voltage change. It is this amplification which amplifies
both the analogue voltage which is being generated as a result of
photons striking and releasing electrons and the noise from all the other
sources.
The signal to noise ratio decreases as we amplify the signal more (due to
additional amplifier noise being added). This results in more noise being
interpreted as image pixels, rather than those directly corresponding to
the real image light derived output.
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