We often read in the photographic literature that it is very interesting to «expose to the right» when shooting in RAW format.
In other words, slightly over-expose his pictures during shooting, then adjust the correct exposure in post-processing (eg in Lightroom).
I would like to share with you an analogy that explains the reasons of this technique.
A digital sensor is an analog-digital converter, that is converting image through the lens into digital information, usable by the electronic circuits of the camera, and more generally, compatible with the numeric world (storage, image processing, distribution,...).
Like any converter, it converts the relevant informations, the image, and adds unnecessary informations, called noise.
To use full capacity of the sensor and to get the best quality image in the RAW file, we can use an analogy with musical recording.
When
recording
music,
they
are
two
concepts,
gain
(recording
volume)
and
distortion.
To
illustrate
these
concepts,
we
can
remember
vu-meter,
which
allows
to
adjust
the
level
of
the
recorded
sound
without
it
reaches
the
«red
zone»,
synonymous
of
distortion.
Thus,
we
will
try
to
turn
up
the
gain
as
much
as
possible
in
order
to
have
the
best
quality
of
recorded
sound.
With
this
operation,
we
aim
an
optimal
signal
amplification
with
as
less
noise
as
possible
(the
best
S/N
ratio).
If the gain is not high enough on record, we will need to increase the volume when listening. In this case, we amplify the sound, but also the noise: the sound can be heavily polluted by noise.
Furthermore,
do
not
turn
up
the
gain
too
much,
because
it
may
generate
distortion.
In
this
case,
the
signal
with
too
high
level
can
not
be
properly
processed
by
the
recorder
(the
signal
is
clipped,
saturated)
and
can
never
be
properly
restored
on
subsequent
listening
(sound
is
distorted).
In photography, it is the same!
The camera histogram controls the exposure.
If the photo is under-exposed, the relevant informations of the image are mixed with noise. When developing the RAW file, we will have to increase the exposure to make up the under-ex, which will raise the level of noise in the image, particularly in low-lights.
On the contrary, if the picture is very over-exposed, high-lights are «burned» (the sensor can not record additional information: we have nothing else but white). When developing the RAW file, we will have to decrease the exposure to make up the over-ex, but the informations are definitively lost (the «burned» areas remain white).
«Expose to the right» its RAW (correctly over-expose his pictures) re-uses the concepts used for sound recording: maximize the recorded informations in order to have the noise level as low as possible in low-lights, while taking care not to saturate the high-lights.
When developing the RAW file, we can restore the good exposure of the picture, but with a better quality than if the photo had been exposed normally, or worse, if it had been under-exposed.
Of
course
these
arguments
are
not
valid
for
JPEG
format.
In
this
case,
as
the
photo
is
«frozen»
in
a
less
rich
format
than
RAW
format,
we
have
to
shoot
with
good
exposure
because
it
is
very
difficult
to
correct
photos
after
they
have
been
under
or
over
exposed.
This shows you one of the advantages of shooting in RAW format, I shall return to this subject shortly.