From: wcharmon (wcharmon@wt.net)
Date: 09/27/02-08:03:17 PM Z
From: Clay Harmon <wcharmon@wt.net>
Date: Fri Sep 27, 2002 8:41:18 PM US/Central
To: alt-photo-process-l@skyway.usask.ca
Subject: Re: digital question (flaunt expertise, please...)
Judy and list:
Sorry for the delay in posting this, but I finally got
around to digging out
the excel spreadsheet for the Yule-Neilsen function which
relates
%black to log reflection density. I will be glad to send the
spreadsheet
to anyone who contacts me off-list. I have also scanned the
graph the
spreadsheet produces so that I can mail an image file of the
graph if
your computer does not have excel. For the insatiably
curious, here are
some selected points from the curve, which assumes a maximum
reflection density of 1.4, which is pretty good for the
matte watercolor
paper on which most of us do our work:
% black Approximate Log reflection Density ( Read
from graph )
0% 0.0
5% 0.03
10% 0.07
20% 0.14
30% 0.22
40% 0.30
50% 0.40
60% 0.53
70% 0.65
80% 0.85
90% 1.05
95% 1.20
100% 1.40
This just illustrates that percentage black is not the same
beast as log
density. So if you want to calibrate your system using the
H&D curve
approach and a reflection densitometer we are used to using,
you
need to do a conversion. So, for instance if your 60% point
on screen
prints to a reflection density of 0.53, you're dead on.
Otherwise, an
adjustment is called for in the negative step. Hope this
helps.
Clay
On Friday, September 20, 2002, at 10:45 PM, Judy Seigel
wrote:
Clay, I don't have an excel spreadsheet.... should I try
some other?
Would this be different/better than simply reading on
densitometer? I'm
still not clear if that reading is as close as it seems to
be -- on
diffusion dither the area does pretty much register as
grey.... I think.
Meanwhile, thanks for suggestion,
Judy
On Fri, 20 Sep 2002, Clay Harmon wrote:
Judy:
If you're wanting to use the step wedge to plot your
traditional H&D
curve that everyone is used to looking at, there exists a
formula for
converting %black to log density (reflective). It's called
the
Yule-Neilson curve and it can be easily plugged into an
excel
spreadsheet to convert %black to reflective density. Here's
a link to a
site that has a .pdf file with this discussed:
www.media.hut.fi/~as75106/AS-75106/harjiotus2/h2e.pdf
Thankfully, it is in English instead of Finnish.
This archive was generated by hypermail 2.1.5 : 10/01/02-03:47:10 PM Z CST