[filmscanners] Re: IV ED dynamic range... DYNAMIC RANGE!

["Julian Robinson" <jrobinso@pcug.org.au>]

At 11:57 08/08/02, Austin wrote:
>Density range is merely the extents that it
>can capture density...but dynamic range is the resolution within that
>density range.

aaaaaaaaaaaaaaaaagh!!!!

NO!

(If you get confused by my first few paragraphs here, please go directly to
the practical example at the bottom - see "PRACTICAL EXAMPLE" below).

DENSITY RANGE is the maximum range of densities that the scanner can deal
with / capture - using any available settings.  You will in general have to
modify gain and maybe other settings and do two different scans to capture
the minimum density and the maximum density that the scanner can record,
but the range between them is the Density Range.

DYNAMIC RANGE on the other hand, is the smaller range within the Density
Range that the scanner can capture AT ONE TIME i.e. dynamically i.e in one
scan.  It is the instantaneous range the scanner can handle.

The reason that Nikon claim better Density Range and not dynamic range (see
Bob's report in the quote at the bottom) is simply because the former is
larger than the latter!  It looks better as a spec.  No mystery
there.  Someone go and ask Nikon if you don't believe me. (The hard part of
this exercise is to make sure you access someone knowledgeable in Nikon).

It is a simple enough concept.  Most explicitly, dynamic RANGE is ***not***
the RESOLUTION, and there is no book or standard that has ever said
this.  This is a confusing statement, and should not be unleashed on people
who are not of engineering background.

I don't want to start the war again, but simply, this statement "dynamic
range is the resolution within that density range" is not true.  To say so
is a bit like saying that a cup handle is a saucer because if you count
them you "always" get the same number -  so they must be the same
thing.  Yes, it is true that almost always anyone who counts the number of
cup handles in a tea set will get the same answer as the person who counts
the saucers, but this does not mean that a cup handle is the same thing as
a saucer.  Furthermore, you will only get the same number IF the cups have
one handle each, and IF these cups use saucers.  These are assumptions and
they are usually true, but are not true in all cases.  You have to be
careful in stating that just because two numbers are usually the same, they
must be measuring the same thing.  It is not true.

Dynamic Range is not a resolution, although the number that you measure for
Dynamic Range will OFTEN and WITH SOME ASSUMPTIONS be the same as the
number you measure for resolution.

Let me rephrase my first definitions, hoping for clarity -

The ***dynamic range*** is the range of densities that the scanner can scan
or discern at one time, which means in practice in a single pass.

The ***Density Range***  is a larger range than the dynamic range and is
defined by the range between -

a) the greatest density the scanner can scan when you set exposure, gain or
other settings to maximise detail in shadow areas, and

b) the smallest density the scanner can scan when you tweak its settings to
maximise highlight detail.

PRACTICAL EXAMPLE
*************************
Here is an illustration as you might measure it in simple, practical terms
on a Nikon scanner.

In the following example, I will use decibels (dB) units.  If you are not
technically backgrounded do not fear, a dB is simply a convenient means of
expressing the levels of things that work logarithmically, like sounds and
light levels.   If a measurement is 1dB higher than a first measurement,
that means it is 1.26 times larger than the first measurement.    So 3dB
means a ratio of 1.26 x 1.26 x 1.26 = 2.0.  And thus a density which is 3dB
greater than a first density is twice as dense as the first density. That
is, it will let half as much light through as the first density.

Let's suppose you have a slide with a nice step wedge covering the range of
densities from near black to near white (clear) on it, in 1dB intervals.
The slide covers 35dB, so there are 35 steps visible.   Step 1 is the
darkest or densest and step 35 is the brightest.

MEASUREMENT 1
First, set the Nikon scanner analog gain control to a low gain, and scan
the slide.  You will get an output scan on which you can clearly see (let's
say) steps 7 to 34 (that is, a 27dB density range).  Because you set the
gain low, you will be able to see most of the bright wedge steps, but below
a certain point everything will appear black - equivalent to losing your
shadow detail on a normal image.  In this case your scanner can not see
step 35, but it does see steps 34 to 7, and then there are 6 steps missing
in the blackness at the bottom.

Your dynamic range is (34 - 7) or 27dB.  (This is equivalent to saying that
the dynamic range is 500 to 1 if you work the maths out).

MEASUREMENT 2
Now set the scanner gain control to a high gain, and do a second scan.  On
your output scan this time you will be able to see, say, steps 2 to
29.  This time you will miss out on the bright-end steps because you set
the gain high and so the scanner saturated or "blew out" on the bright part
of the test slide.  But the range it collected was still 27dB - the same
dynamic range.  (Dynamic Range this time = 29 - 2 = 27)

So you have measured your dynamic range twice, and it was 27dB.

Now, what is the Density Range?  It is the range from the brightest step it
could record to the darkest.  The brightest was in the first scan, and was
step 34. The darkest was in the second scan, and was step 2.  So the
Density Range is 34-2 = 32 steps - that is, 32 dB.

Voila.  Your example Nikon scanner has a Density range of 32 dB and a
Dynamic Range of 27dB.

This illustrates the correct distinction between density range and dynamic
range.  Do not be confused by anything else!

Because the Density Range is higher, this is what Nikon like to
quote.  Wouldn't you, given that 99% of us are too confused to know the
difference? It is also a  performance measure that is arguably as useful as
the dynamic range in determining how good the scanner is.  They measure
different things but both are useful.

Julian

PS the NIkon scanners are the perfect example for this illustration,
because they have actual, accessible, analog gain controls.  Most other
scanners hide the analog gain variation inside some kind of auto-exposure.


>Charlie,
>Nikon claims better 'Density Range' for the LS4000 not 'Dynamic Range'. I'm
>sure Austin would tell us which is correct!!! I've forgotten already.
>Bob Frost.


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