Re: scanner density-value relation

----- Original Message -----
From: "Ryuji Suzuki" <rs@AgX.st>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Tuesday, March 08, 2005 11:56 PM
Subject: Re: scanner density-value relation

> From: Richard Knoppow <dickburk@ix.netcom.com>
> Subject: Re: scanner density-value relation
> Date: Tue, 08 Mar 2005 21:54:21 -0800
>
>> I wonder if flare is a factor. The test is to mask off
>> the
>> area of the step wedge so that the rest of the field is
>> not
>> illuminated.
>
> In his case, maybe, because he reported such a drastic
> sensitivity to
> location/direction. Mine is very different, as I tested
> with a step
> tablet and also with large pieces of ND filters. I also
> compared with
> the reading of a small piece of opaque stuff, forgot what,
> but
> probably a metal foil sandwich-glued with black paper, in
> which some
> papers and films are packaged. So I don't think it was
> flare or
> internal reflection. If my scanner were that bad, I'd call
> customer
> support for refund before I write here :-)
>
> I remember writing some MATLAB codes to load the raw TIFF
> and do
> statistical analysis, curve fitting, etc. but lost it
> somewhere when I
> switched my computer. Now I'm interested in making a color
> densitometer again to quantitatively record the effect of
> emulsion
> formula, developer formula and toning to the image hue.
> Typically blue
> and red reflection densities are used for those studies
> but I don't
> have a color densitometer. With advertised Dmax of 4.0
> (say that's
> bogus and the linear portion goes only to 2 point
> something -- Warning
> -- it's a number I just made up) it's still useful for
> routine
> densitometry of pictorial negatives and prints.
>
> By the way, if the quality of software is entirely
> disregarded, are
> Perfection 4990 and Canoscan 9950F very closely comparable
> in terms of
> image quality as far as b&w negs and prints are concerned?
> What about
> usable Dmax?
>
> (Those online reviews using dark background with fine
> patterns as the
> test target are not very useful for this purpose because
> they can't
> separate the noise issue and the linearity issue.)
>
> --
> Ryuji Suzuki

  A simple densitometer can be very useful, even for some
color work. I have an ancient Weston densitometer which uses
a large selenium cell similar to, but larger than, those
used in Weston exposure meters. It has a shaped pole meter
so it gives reasonable readings up to a density of 3.0 on a
single scale. This is higher than usually encountered in B&W
pictorial negatives. It allows me to get some idea of the
contrast of negatives, their fog level, and general curve
shapes. I would love to have a "real" densitometer but that
will have to come in the future. It seems to me that a
decent scanner ought to be quite servicable for making
sensitometric measurements. For that matter reasonable
measurements of density can be made visually by comparison
to known neutral density filters.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com