8 gives 256 tones including black (0) and white (255), while a depth of 14
gives 16,384 tones including black (0) and white (16,383).
As stated earlier, the amount of charge in a photosite’s well is
determined by how much light has fallen on the photosite. It is possible
for a well to fill before an exposure has completed and this can lead to
two conditions. In some sensors the well will fill and overflow, its charge
cascading into adjacent wells. The architecture of an image sensor
normally dictates the direction of flow to be constrained to the next well in
the same column of photosites.
The effect on the final image is that a line artefact appears from very
bright sources (e.g. a bright star). Most general-purpose sensors avoid this
issue by the application of an anti-blooming gate (ABG) to each photosite.
This effectively caps a well when it gets full, preventing the occurrence of
the cascade artefact.The function that allowed this to happen was the introduction of instant
image review, a simple yet powerful facility that allows you to see
what your camera has recorded as soon as an exposure is complete.
With older film technology, the same process required the film to be
removed from the camera and developed.
The instant review creates a rapid feedback loop allowing you to take
a shot, review it and make the necessary adjustments to the camera
settings to correct any issues identified during the review.
Apart from the obvious benefits of being able to immediately
correct poor setting choices or framing errors, the long-term benefits
of interactive digital imaging also help nurture a better feel for your
camera’s settings and what they actually do.
As an added bonus, most camera images store important setting
values in the file header of the image. Should you forget what settings
you used for a certain shot, it is therefore possible to use a simple
image utility to read and remind yourself of these values.Digital Sensor Basics
The term digital camera encompasses many different types of device
but they all have one thing in common: an image sensor. The image
sensor can be thought of as a grid of light-sensitive "photosites". When
light falls on to a photosite, a charge is created and stored; the more
light that a particular photosite encounters, the higher the charge.
At the end of an exposure, the grid of photosite charge values
are read off and converted to numbers. Interpreted by a computer,
these numbers represent the tonal image that the sensor saw, and
reconstructed in the same grid formation, can be used to reconstitute
the image. The amount of charge a photosite can hold is called its
"well depth". The interpretation of the amount of charge held is done
by defining zero charge as 0 and a full well by a value equal to 2n–1 (2n
means 2 multiplied by itself “n” times where n can be 1, 2, 3, 4, etc.), n
being known as the sensor’s bit-depth. The higher the value of n, the
greater the maximum number becomes, and this translates into a
greater number of tones in the final image. If n were trivially small, at
say 2, a photosite’s charge would be able to represent values between
0 and 3. The value of 0 is used to represent black while the maximum
number represents white, so for n=2 the sensor could represent black,
white and two tones in between.
A bit-depth of 2 would be very tonally limited, and modern sensors
have far higher values of typically 8, 10, 12, 14 and 16. A bit-depth of[1] A digital single lens
reflex (DSLR) camera.Two of the most common questions from modern newcomers
to astronomy are “Which telescope should I buy?” and “How do I
take pictures of the night sky?”. The second question is relatively
new and is the direct result of the success of the digital camera,
a device that has revolutionized astrophotography over recent
years. For many, the lure of being able to capture beautiful
images of the stars and planets is difficult to resist.ASTRONOMY WITH YOUR CAMERA
1Astronomer Book