Should I use Auto Adjust?
Lots of SilverFast users shy away from the Auto Adjust
tool
thinking that it
is going to damage their images. Maybe it will
and maybe it won't - but the USER has control. By getting a basic
understanding of how
Auto Adjust works we can save ourselves a lot of agony.
LaserSoft have provided an exceptionally powerful tool and we
should learn to use it. Note that in the
screenshot shown below I show Auto
Contrast as OFF - this is my normal approach - others
are free to choose otherwise (Experiment!). The settings shown
inside the red boundary are the KEY to ensuring that minimal data
is lost whilst getting the best from a well exposed and normal
toned original. Read
Chapter 5 of the user manual, read it again, then configure
the Auto section of the SilverFast Defaults window correctly and you won't be sorry.
Tips
-
setting the highlight offset to 0 is
not a good idea since the highlight pixels will be clipped
to level 255 upon application of Auto Adjust.
-
setting the shadow offset to100% is
not a
good idea since the shadow pixels will be clipped to Level 0
upon application of Auto Adjust.
Auto Defaults (Figure 2)
The Histogram
The following screenshot shows the histogram for
my sample image after pressing the Auto Adjust button (using the settings I suggested above). Simply pressing the Auto Adjust button
will mean that the image highlight and shadows will be optimised
such that the spread of image data is more evenly distributed
(figure 3). Notice how the shadow and highlight points in the
histogram are all equal which means no colour correction has been applied
to the image; I'll explain why in a moment.
Histogram after Auto Adjust
(Figure 3)
We can also see from our previously created
densitometers that the values for the dark and light points have changed
after applying the Auto Adjust
tool.
If the original pre-scan was assessed as being
dark/light by SilverFast then an appropriate correction will
automatically be applied (figure 4). In the screenshot shown below I show
an adjustment of -4 (brighten the image). Positive numbers
will darken the image.
Histogram after Auto Adjust
(Figure 4)
Important note about Auto
Adjustment tool!
For SilverFast users with IT8 calibration or a
customised ICC scanner profile SilverFast will ignore any colour
casts it finds in the pre-scan and so Auto Adjust will simply
correct the tonal range of the image. If on the other hand you are
using SilverFast without the benefit of calibration then
SilverFast will automatically remove any colour casts found in the
image (not a good
idea for sunsets). The user can preset the amount of
automatic colour
cast removal by inserting a value into the Color Cast Removal popup
shown with a green asterisk (see
Figure 2 above for an example). The default value of 100% will
completely remove the colour cast an a value of 0% will leave the
cast untouched. Alternatively we can hold down the Shift key when pressing
Auto Adjust
and the colour cast will be retained in all its glory. We can identify whether
SilverFast will or will not automatically apply colour correction by
checking which form the Auto Adjustment (aperture symbol)
takes in the Adjustments toolbar (see Auto Adjust tool
variations on page 1).
Colour cast removal can be
fine-tuned at any time using the Color Cast Removal slider.
The screenshot below shows how smart SilverFast is at removing
colour casts using the Colour Cast Removal slider. I push one
slider and SilverFast decides which channels, if any, require
adjustment to neutralise the cast. As mentioned above; no colour cast removal is Zero
% and complete colour cast removal is 100%. In the example below I show 75% of
the cast has
been removed. Remember not all colour casts are bad, some are an
inherent feature of the image (sunset).
We can also manually adjust the vertical sliders
for each channel by simply grabbing each in turn with the mouse or
as a group (highlight, shadow or mid tone groups) by grabbing the appropriate
triangle directly under the histogram.. Experiment in this dialog and you will learn lots of neat tricks to
refine your images.
So how much damage has been
done to my Image?
Are you worried about trashing important image data?
Do you need to know how the image data will appear in Photoshop?
With the Levels Histogram open simply hold down the Alt/Option key and watch the data in the histogram
spread. It's probably better to use the combined histogram for
this test as it will more likely be comparable to the Photoshop
levels dialog.
Notice that through careful choice of values for the
Shadow and Highlight Threshold in the Auto
Defaults dialog (figure 2 above) I have managed to ensure that
no clipping occurs at the end points. In this example the end points
were defined as 98% for the shadow which equates to Level 5 and 2% highlight
which equates to Level 249. If we can manage to avoid clipping at
the scan stage we can be confident that saturation of individual
colour channels has also been avoided
Note that any spikes (there might not be any) in
the histogram represent values that have been rounded (made the
same) and usually results from the high to low bit conversion. Gaps
in a histogram represent data values that have been lost. In
simple terms pressing the Alt/Option key when viewing the histogram will
represent the 8-bit data being output to Photoshop. It's also
important that we remember EVERY edit and conversion will loose data. Even applying the basic and very
necessary gamma adjustment to the raw scan will loose data.
Conclusion
So what have we learned from this tutorial:-
-
The basic tools
-
How to assess the Pre-scan
-
Find and store the location of darkest and lightest
pixels
-
Configuration of the Auto Adjustment
tool
-
How to use the Levels Histogram tool
-
How the maximise the range of the scan
-
How to brighten/darken the scan
-
Automatic and manual Colour Cast Removal
-
How using the Alt/Option key and Levels
Histogram we can preview
the "edited data" being sent to Photoshop.
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