Hi Erik.
Erik Kaffehr wrote:
> .......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.
Acer claim a 3.2 D range for the Scanwit 2720, and I've verified that this is
actually acheived in practise.
I used neutral density filter gels in 0.3D steps down to 3.9D, and the variation
in returned RGB level bottomed out at 3.3D. The Dmin seen by the scanner is about
0.15D (about average for the base+fog of a real transparency), giving a density
range of 3.3-0.15 = 3.15D.
I did the same test with the 2740, which has a 14 bit A/D, and got virtually
identical results.
ie. nearly no discrimination of density change below 3.3D
Acer, quite rightly, are still only claiming a 3.2D range for the 2740, and not
even making a big deal of the 14 bit A/D fitted.
This leads me to the conclusion that the limiting factor determining true dynamic
range is the CCD sensor, and not the A/D converter chip. (Acer's R&D confirmed
this, when I queried the lack of dynamic range improvement in the 2740)
There is a consideration of Infrared fog, or breakthrough, and the reduction of
real contrast by the optical system, (the system OTF), but this is a different
issue, and doesn't change the fact that a real density range of around 3.2D can
be captured at the film plane.
It's easily possible to increase the density (Dmax) seen by a scanner by simply
increasing the exposure time or the light source intensity, but this leads to
saturation and possible 'blooming' of the CCD at the top end.
The (real) dynamic range is what is important, not a spurious Dmax figure as
quoted by some manufacturers.
However, if scanner makers really wanted to confuse the issue, they could state
their dynamic range figures in dB, as per the CCD data sheets. (using power dB, a
bit naughty!)
Regards, Pete.
