Scanner Gamma is sometimes referred to as Scanner Tone
Response and most scanner
applications will include a facility for altering it. The
following tutorial makes extensive use of LaserSoft SilverFast, however,
even if you don't use
SilverFast I recommend that you read through the tutorial and try applying
it to your preferred scanner software. With most scanner applications gamma
will operate in a broadly similar
manner therefore most of what follows should be easily read across to
offerings from Polaroid, Nikon, Minolta, Epson et al.
For SilverFast users who are still unfamiliar with the term Gamma-Gradation
(gamma for short) it can be found in a dialog labelled Defaults
(click the Option button to open) and more specifically the General Tab.
The SilverFast User Manual describes Gamma broadly as
follows: This function adjusts the general brightness of the image for
midtone and shadow. In general, a gamma of 1.6-1.8 is recommended for
reflective media. For transparency, the gamma is normally higher than for
reflective media with values between 1.8 and 2.0 being normal although
values up to 2.2 are perfectly acceptable. Lower values will tend to
result in a darker more contrasty preview than higher values. Choosing a
higher value also means that the shadows will be more open than for lower
values; the image a bit brighter, however, image contrast will tend to be
lower. That said some experts do recommend values as high as 2.6 and
higher.
Through experimentation I have standardised on a value of 2.0 for
my Polaroid SprintScan 120 and 2.2 for my Polaroid SprintScan 4000 and
Nikon 4000ED. This is not to say that the higher gamma values aren't
equally valid just that I personally don't find them to offer any
significant improvements to my scans.
The different values likely come down to how the scanner hardware is
internally calibrated by the manufacturer. In my case choosing these values also
means that an image will preview almost identically on each scanner. The
next section discusses how we go about achieving the optimum
Tone Response Curve (shortened to Tone Curve) or gamma-gradation value.
Note: the values quoted below for
patch GS11 on IT8 targets are specific to transparency type from
manufacturers such as Fuji, Kodak and Agfa. Some reflective type targets differ slightly and so the values quoted for GS11
should be read as GS10.
Determining the optimum Gamma-Gradation value
The SilverFast User Manual goes on to say: "Check
that your monitor is set to the correct gamma in order for the tonal
values to be displayed evenly. For best results scan a grey calibration
strip and adjust your gamma accordingly". I think this is the point
at which the wheels come off for a lot of folk. The impression being given
is that there is a direct linkage between the scanner gamma and monitor
gamma, which is not really the case. Your monitor should be calibrated in
the normal way and independently of your scanner - a gamma value of
between 2.0 and 2.2 is accepted as being a good approximation of
human vision. Gamma 1.8 as used by Apple Macintosh computers is really
only used for legacy images. Many Mac users (myself included) have
standardised on 2.2 and that is now my recommended value for all systems. There is a lot of material and guidance elsewhere on
this web site on how to calibrate the monitor so I won't repeat it here.
The purpose of scanning the grey calibration strip (you're better to use
an IT8 calibration target for slide scanners) is to find a the ideal
Tone Curve or gamma-gradation value for your particular
scanner. The simplest approach is to adjust the gamma value so that the grey patch
GS11 on the
IT8 calibration slide has an average RGB value in the range 100-115. This
means scanning the IT8 at different gamma values then selecting the one that is closest to
the optimum. You should also find that the average RGB values for patches
GS0 (white patch) is in the range 235-250 and GS23
(black patch) is in the range 10-25. In each case it is better to
avoid values outside these ranges.
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IT8 Calibration Target
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The following series of screen-shots shows the results of my effort to
demonstrate the technique and should give you some idea as to how
increasing the gamma value will cause all of the values from near black
through to very light grey to become lighter. The actual preview image
became progressively brighter but less contrasty as I increased the gamma
value. The large peak around the centre is actually the grey boundary area
of the IT8 and not patch GS11. You should bear in mind that nothing is set
in concrete and if you prefer the results with a lower/higher gamma value
then use it instead. However, before committing yourself to this value you
might wish to scan an image that has real blacks, real whites and a good
range of tones between then check the histogram. If the histogram shows
any clipping of highlights or shadows you've chosen a gamma value that will likely cause you
many headaches in the future.
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Gamma = 1.8
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Gamma = 2.0
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Gamma = 2.2
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This procedure is akin to calibrating the brightness and tone
response of the scanner and as with all calibration processes the idea is
to bring the device to a predefined condition or setting and leave it
there. All future scans should be undertaken using your preferred gamma value with specific image adjustments being carried out
using the Colour and Tone editing "tools" provided with the scanner
software.
The effect of Gamma-Gradation on IT8 Calibration
The procedure described above will have no effect on how the scanner
reproduces colour. If the scanner reproduces colour badly you will need to
take further steps to establish the cause of the error and then alter your
workflow to eliminate
it. The most common approach to obtaining good colour output from a scanner
is IT8 calibration and LaserSoft offer this feature as an optional extra. However,
if you already have a third
party profiling application from companies such as
Monaco Systems
or Gretag Macbeth
these will do fine.
Those of you reading this who already own a scanner profiling
application such as those mentioned above will be wondering if the above
procedure for determining the ideal gamma is either necessary or
desirable. The simple answer is NO and Yes in that order! The reason it's
not necessary is the IT8 calibration will always reproduce grey-scale
patch GS11, etc as per the associated reference data file
irrespective of what gamma value you choose to make the original scan
with. In effect the IT8 calibration process will produce the optimum Tone
Curve. However, should you choose to use an alternative gamma value at
some future time and still retain the original calibration profile you should not expect
images to match in terms of brightness and contrast. The calibration
profile is specific to the gamma value we used when creating the profile. The
following series of screen-shots show the histograms for an IT8
calibration slide that has been scanned using profiles created at
different gamma values. Notice that they are identical; meaning that no
matter which gamma value I choose to create the profile with the results
are always the same; that is so long as I do not alter the gamma value
afterwards. Unlike the examples shown in the first section IT8 effectively
prevents the gamma value altering the brightness and
tonal response of the scanner since they both become a function of the
calibration profile.
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Gamma-Gradation = 1.8
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Gamma-Gradation = 2.0
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Gamma-Gradation = 2.2
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So now we're left with why it might be desirable to preset the optimum
gamma value. The reason should be obvious in so far as the less
work the profile has to do to achieve a target value the less data is lost
from the image as a consequence. A simple example should help
explain: say we preset the gamma value to 2.2 - we see from earlier
example for the SprintScan 120 that the GS11 patch will have a value greater than the desired
value and so the profiling application must make a correction to bring it
back to desired value. In making the correction some data will be lost and
since a fair amount was lost when the original gamma
curve was applied we can ill afford to loose more going backwards.
Also, it should not have escaped your noticed that once IT8 calibration
comes into play the ability to adjust the placement of the mid-tone value
is lost, however, the shadow and highlight values are more accurately
placed, albeit fixed. The downside of this accuracy should be plain to see
from comparing the the two sets of screen-shots. Each set used an
identical slide (Fuji IT8 calibration slide) and we can see from the first
set that the shadow and highlights are never in danger of being clipped.
Once the IT8 calibration is activated we find that shadows in particular
may be prone to some clipping. Using real world images I've not found
shadow clipping to be a problem with either the SprintScan 4000 or 120 but
I was less fortunate with my Nikon LS30. I think this was more a
characteristic of the Nikon hardware than a failure of the profile or
SilverFast. Modern 12 and 14 Bit film scanners such as the Nikon 4000ED
are capable of
rendering highlight and shadow detail in all but the most extreme cases.
