Digital BW, The Print

But I have seen many reports 
> of other methods like your's. 

Yes...we report that because the simple method works.

In my opinion Epson QC tries to keep the 
> focus at 1 mm above the glass as the film in the carriers is at that 
> height. 

I don't know how "opinion" is relevant. If you have Epson
documentation about "1mm", please share it. In MY "opinion," its 
urban legend :-)  As likely the APPROXIMATE 1mm is just a convenience
 for injection molding...nothing to do with focus, since focus isn't
critical, as you point out here:
 
> in practice, it is hard to notice where it exactly is because the
DOF is 
> so extreme.

Yes..."in practice" its not significant in large film (unlike 35,where
we work with sparse film information, large film allows a multitude of
sins).  

A scanner isn't like an enlarger or contact printer. It's a reader: it
reads and reports densities. When a point source is used an optical
compromise is evidently introduced...the reason Minolta scanners do so
much better when manually focused than autofocused.

> I wouldn't dare to claim that you can do without a mask. 

Try it.  There may be an effect around the edge, but even that
probably won't be noticable unless that edge is buckled or torn. 

Precise location of Silverfast's dotted frame is what's critical
...light frome elsewhere, such as around the edges, is simply rejected
digitally.

Especially not 
> on the 3200 that has a fixed lightsource over the entire length of the 
> scan area. 

The moving source may somehow account for the 4870's slight sharpness
advantage over the 3200 with 35mm negs, but it's proportionately less
significant with large, old negatives and common enlargement sizes. 

For enlargement to a given size, a 4X5 (5") negative would get only .3
(30%) of the benefit that a 35mm (1.5") negative would get, switching
from a 3200 to a 4870. Might be visible in a mural, if that's relevant. 

How a densitometer can be 
> used for that purpose is a mystery to me.

Understood. Its a mystery due to careless word usage by marketers. A
densitometer is simply a device that quantifies density, measuring
transmitted or reflected light. Many devices can do that...even common
spotmeters, if used with a stable light source.

Incredibly accurate densitometers were in use fifty years ago by the
expert lithographers. Good photolabs started using them in the
Sixties. Until their recent sloppy decline, minilabs routinely used
them to monitor process strips.  

The practical use of a densitometer for our discussion would be in
print evaluation (Forum centers on prints, visual objects, not digital
readouts) .. we'd begin by measuring blacks around edges, since
contrast and curves are so easily tweaked in Photoshop. 
 



 
> 
> Ernst