Can someone help me here with some basic facts regarding this
dynamic/density range business?
I am having a fundamental problem comprehending why the number of bits is
even vaguely related to any supposed density range. I understand the maths
quoted here and in many other posts, but fail to understand why the fact
that the ratio of smallest bit size to largest number represented should be
related to density range.
For example, I could have a density range of 1000:1 - ie 3.0 on the log
scale. What is to stop me representing this by 4 bits or instead by 40
bits? The only thing that changes is the resolution.
In the former case, log10 (2 to the power 4) = 1.2 = "so-called calculated
density range"
In the latter case, log10(2 to the power 40) = 12.0 = "so-called calculated
density range",
but in both cases the actual density range represented is still 3.0.
The difference of course is the resolution...
In the former case, there are only 16 levels between darkest and lightest
density.
In the latter case, there are about 10^12 i.e. 1,000,000,000,000 levels.
I cannot shake off the belief that the number of bits affects nothing
except the resolution with which the densities are recorded, and has
nothing whatsoever to do he usable density range.
I thought that the density range is the ratio of:
[the brightest accurately measured value through clear (fully exposed)
slide film or orange mask (unexposed) neg film] vs
[the darkest accurately measured value]
which has nothing to do with the number of bits in the D/A.
The brightest value will be determined by the level that can be read by the
CCD with reasonable (predictable) linearity.
The darkest value will be determined by noise performance of the CCD.
What am I doing wrong?
Julian
At 17:44 10/01/01, you wrote:
>Hi!
>
>A 14 bit number means a range 2 raised to the power of fourteen, that is
>16384.
>
>Density units are log 10 so we get log (16384) -> 4.21
>
>A simpler way:
>
>1 bit means essentially one one aperture stop which is 0.3 Density units (log
>2).
>
>14 * 0.3 -> 4.2
>
>Or you could also say that the range is 14 aperture stops.
>
>This is more than the density range of any film...
>
>Practical measurements on existing scanners seem to indicate that the real
>dynamic range (including CCD, elektronics, external light internal
>refkections seem in the order of 2.5, that is 8-9 aperture stops.
>
>Regards
>
>Erik
>
>
>
>
>On Tuesday 09 January 2001 19:05, you wrote:
> > > In summary, dynamic range is just another way of saying how
> > > many bits the A/D converter uses:
> > >
> > > 10 bits = 3.0
> > > 12 bits = 3.6
> > > 14 bits = 4.2
> >
> > Would you please explain this more? What is the source of the information,
> > or the algorithm, you used to come up with these numbers?
>
>--
>Erik Kaffehr [EMAIL PROTECTED] alt. [EMAIL PROTECTED]
>Mariebergsvägen 53 +46 155 219338 (home)
>S-611 66 Nyköping +46 155 263515 (office)
>Sweden -- Message sent using 100% recycled electrons --
Julian Robinson
in usually sunny, smog free Canberra, Australia
