Quick executive summary :
If your scanner has an 'auto exposure setting', use it after
you've defined the area to scan. Most of the time, this will make
your image very much better, and will be sufficiently good compared to
what you'd get by setting the levels by hand.
Have a look at these two
images. The same stamp, and even the same scanner, but what a
difference in image quality.
Do you notice how the image
on the left looks a lot softer than the image on the right - it has no
contrast and neither does it have much brightness. The reason for
this primarily to do with proper use of the 'exposure' control (or
whatever it is called) in your scanning program.
Here in the exposure
control of my scanning program you can see a graphical representation of
the stamp image's range of light/dark values.
Notice how there is a bit
of space with no values at either end - this is perfectly normal.
Ignore the way I have the sliders set, just look at the distribution of
the brightness values.
The first scan was taken
essentially with no change to the image as it was first sensed by the
scanner. The spread of color values remained similar, but not
exactly the same (due to inconsistencies in the scanning process and
various other things), as you can see in this Photoshop Levels display
Then I clicked on the Auto
button, which moved the highlight and shadow sliders to their new
positions, as you can see below, and scanned again.
This has made the picture
very much clearer, because the color values are now more evenly spread
from light to dark. As you can see from the Adobe levels window with
the final image, the brightness values now flow pretty much all the way
from minimum to maximum
This is an example of an
image where the 'auto' setting worked close to perfectly. There are
two situations when you might want to override the auto setting.
The first is when the auto
setting 'cuts off' too much image data. Here are two more scans to
illustrate what happens when you 'cut off' image data. I've used the
150 dpi images this time, because the effect is a bit more subtle.
Do you notice the
difference? At first you might think that the image on the left is
better, but it isn't. It is more contrasty, for sure, but you've
lost some of the detail in the shading - look for example at the sky or
the man's collar or his rolled up paper - some of the lighter shading has
been entirely lost into the plain white. By the same token, some of
the darker shading has been lost into the solid brown or black.
So, if you have a situation
where the auto setting moves the minimum and maximum into the actual area
where there is detail, this can be a bad thing. Look again up at the
second 'Exposure Adjustment' image - this is close to perfect, with the
minimum and maximums set almost exactly where values first start to appear
on the image. If the two pointers had moved closer together, then
the image would become progressively more like the scan sample on the left
The second situation when
your image doesn't have a standard range of colors from (close to) white
to (close to) black. Maybe it is a very old stamp and the white
paper has faded to brown. Or maybe it is a stamp with a few colors
printed on a predominantly black background. Here is an example of
such a stamp.
Notice how the image on the
left doesn't have as strong a black as the image on the right. The
auto-exposure has tried to 'average' the intensity in the image, and so
have over-compensated for the fact that it is supposed to be a very dark
image, making the colors much brighter and the background much lighter.
The image on the right was scanned with the same settings as the other
'correct' images have been scanned on this page.
Now look at the graph
showing brightness for this image. As you would expect, it is all
concentrated in one small area of darkness, and much of the graph has no
values at all.
I have the sliders
shown in the above image close to where the auto exposure setting would
put them, but this makes the image too light. You need to move the
right slider out to reflect the fact that the lightest thing in the image
is not as light as a white piece of paper.
Lastly, to conclude this
discussion of exposure settings, here is a 'torture test' example where I
scan a piece of pure black and pure white, side by side. Here are
What do you notice about
these two images? Do you notice that the one on the left has a dark
grey rather than black, and also has a light grey instead of a white?
Can you guess why that is?
Let's look at the exposure
graph for this image.
As you'd expect, it shows
some values on the far left (dark) and the far right (light) and nothing
at all inbetween. I've also left the sliders where the auto setting
suggested. Do you think the auto is correct or incorrect?
This is introducing you to
one last subtle and hopefully not too confusing point. These two
images - the back of a Scott mount for the black and a piece of 'high
white' paper for white - are, in theory, close to pure black and pure
white, and this means instead of giving a range of white and dark values,
they actually should just have single lines for 'all black' and 'all
white'. The wider spread of values is (sort of) 'wrong'. If we
scan with the two sliders at each end, to include all values (like I've
been saying you should!) then the image on the left above is what we get -
instead of a solid punchy black and a vivid pure white, we get a range of
dark greys and light greys.
This is a case where you
actually want to set the sliders so that all the values are outside rather
than inside the area between them. This means that all the values
the scanner sees as 'reasonably black' get translated as 'full black' and
all the values the scanner sees as 'reasonably white' gets translated as
Have you understood this
page? Please let me know so I
can add to or change it as necessary.
The issues are indeed
complex, but 99% of the time can be reduced to a simple three step process
First, select the
area to be scanned
Second, use the auto
exposure to recommend values
Third, think about
any special issues such as pure white and black or an unusual image and
adjust the auto exposure setting to suit