Digital BW, The Print

Bill Morse wrote:
> Hi Bruce-
> You're right about the Aztek/Howtek thing, and it is too bad it doesn't work with other scanners. As I understand it, the reason is that the software can directly control those scanners at the hardware level, which is not possible with other scanners.
>
> And yes, scanning in 16 bits you can pull an amazing amount of information our of a difficult negative. In fact, that's why I was so skeptical at first- I thought DPL was just scanning in 16 bits and calling it something special. It wasn't until I compared two scans of the same negative, one "straight" in 16 bits, and the other using the hardware control capabilities of DPL, that I saw how much more was possible with DPL. But again, those advantages are most apparent with difficult film.
>
> Bill

My understanding is that most (all?) drum scanners have the capability 
to have their hardware log amp circuit limits set by the software that 
runs the scanner. That is, you set your black and white points in 
software, and the software tells the scanner hardware to set the 
appropriate limits for the log amp circuits. What this does, in my 
understanding again, is to tell the scanner to translate the density 
range (between the black and white points you specified) to the digital 
range of the ADCs (typically 12 bits in older scanners, newer scanners 
are often 14 bit or even 16 bit).

In contrast, your typical consumer flatbed scanner doesn't have this 
capability to sample a user specified density range. Instead, the 
density range is usually fixed at the maximum for the scanner in question.

This isn't terribly important if you are scanning trannies because 
trannies tend to spread their image data across their available density 
range. This characteristic of tranny films is one of the reasons one can 
profile a scanner when scanning trannies -- the density range doesn't 
vary much between trannies.

But if you are scanning negative films, this is more important. The 
density ranges for negative films can and does vary considerably from 
frame to frame. If for example you scan a B&W negative that has a 
density range of 1.2, you can easily see the advantages of setting the 
scanner's log amp circuits. Scanning in a fixed density range (say, 3.2) 
means you end up with a file that is more than half full of zeros with 
the image information compressed into a small part of the file. 
Correcting this (either in scanner software or Photoshop) often results 
in the loss of subtle tonal detail and smooth tonal transitions. For 
example, you loose the faint detail in those clear areas of the film.

At least that's my understanding of the theory. It's one of the reasons 
drum scanners do such a fine job pulling detail out of the difficult 
parts of the film (the extreme ends of the density range).
--
Bruce Watson