RE: filmscanners: Nikon MF LED light source...

[Julian Robinson <julian@austarmetro.com.au>]

I think I almost agree with you here Austin (heaven forbid!) but the 
problem is that the only logical reference when Dmax is quoted on its own 
is against full transparency, as you state - i.e. no film, nothing in the 
way of the path betw the light source and the detector.  Trouble is there 
is no guarantee that the scanner is not in saturation at this level, in 
which case Dmin is not 0.  But this would not be stated by the manufacturer.

So I don't agree that if a manufacturer states a Dmax figure on its own, it 
necessarily means that this is the scanner's dynamic range - it is very 
easy to steal some dB in the figure by not owning up to a Dmin of a few dB 
which is perhaps more likely than not.**

A different example is the Nikon scanners with their analogue gain which 
are capable of turning up the wick on their LEDs or the gain on their amps 
so that you can see into really dark images.  Hence they could quote a much 
better Dmax (relative to 0) than their competitors (those who don't have 
analogue gain control).  But when you do turn the gain up like this, all 
your highlights blow out impossibly, in other words it is not able to 
handle the RANGE from Dmax to 0, that is, Dmin is not = 0.  But the Dmax 
figure is still valid as a density figure which the scanner can "see 
through" accurately, although it has nothing to do with the dynamic range.

I think the example about the Nikons shows that there *IS* some value in 
DMax  without quoting DMin associated with it, but it doesn't tell you 
anything necessarily about dynamic range, which is the point I think Tony 
was making.

And there is the additional point that *dynamic* range is not necessarily 
the same as *static* (my word because I can't think of a better one) or 
absolute density range.  Dynamic range is the Dmax-Dmin that can be 
recorded at one time i.e.  more or less, in a single scan.  But the total 
density range is the maximum density which can be handled under one set of 
circumstances minus the minimum density that can be handled (and doesn't 
cause saturation) under another set of circumstances.

Once again the Nikons are the example - you can measure a Dmax with the 
gain turned right up which will be 2 stops or so better than with the gain 
at 'normal'. (6db difference I think).  But near-clear highlights will not 
be usable at that setting.  So assuming that with the gain at 'normal' the 
Nikon can record near-clear highlights, then we have two settings...

1.  With gain = 'normal', Dmax(1) = x, Dmin(1) = ~0;
Dynamic range = x - 0 = x dB

2.  With gain = 'max', Dmax(2) = x+6. Dmin(2) = ~6;
Dynamic range = x+6 - 6 = x dB  (the same figure)

But the STATIC or ABSOLUTE range of densities that can be accurately 
measured by the scanner is ...

Absolute range = greatest density that can be measured in any circumstances 
minus minimum density that can be measured in any (other) circumstances 
which (in our example)

=  Dmax(2) - Dmin(1)
= x+6 - 0
= x+6 dB.

So the absolute range is validly 6dB better than the dynamic range.  And as 
I think everyone agrees, both figures need a relevant Dmax and a relevant 
Dmin to be meaningful.

Julian

**In any case as we know and has already been discussed many times on this 
list, the **quoted** dynamic range is usually based on the num of A/D bits 
and so is not related to either Dmax OR Dmin in any case!



Austin wrote:
>DMax is of no value what so ever unless there is a DMin associated with it
>(and vice versa).  Typically, DMin is assumed to be 0 when DMax is used by
>it self.  DMax is equal to the dynamic range if DMax is referenced to a DMin
>of 0.  Plain and simple.  Having a DMax of 4 and a DMin of 1 is exactly the
>same dynamic range as a DMax of 3 and a DMin of 0.  The dynamic range is
>still 3.  All density values are ratio values, and are therefore relative,
>and all you need to know is what the number is relative to.  Without a
>relation, density values are useless.  Dynamic range does not need a
>relation, as it is a "range".


Julian Robinson
in usually sunny, smog free Canberra, Australia