[alt-photo] Re: Alt-photo-process-list Digest, Vol 318, Issue 1

Sat Dec 11 16:22:08 GMT 2010

Hello Peter,
The scanner is a useful tool but it I've been mislead by it. Here is the
lesson I learned.
Stouffer reflectance and transmission step wedge scans can be opened in
photoshop and K values read for each step. The tricky part is that these K
values, if plotted with density values appropriate for each step of the step
wedge, do not correspond. So a simple calculation from Photoshop K to
density isn't accurate. Here is an example from a palladium print of a step
wedge measured both by an Eye-One spectrophotometer for reflection density
and by a V700 flatbed scanner (I compared two different V700 scanners and a
Canon flatbed and got very similar results with each, so it is not just my
scanner acting up). The reflection density values I obtain from the Eye One
match a calibrated Stouffer reflectance step wedge nicely. So I have some
confidence that this plot isn't a red herring:
http://img824.imageshack.us/img824/1743/8412nocurvescanned.jpg
For the above plot the starting and finishing photoshop K values and
reflection density values were set to start and finish at the same points to
normalize the curves. You can see the step wedge numbers on the x axis 0-21
and the %K and reflection density values are overlayed on the Y axis.
A correction curve derived from the scanner vs. the densitometer would
clearly yield different results if applied to the image in photoshop to
create a digital negative let's say.
There is an easy solution if you want reflection or transmission density
values from a flatbed scanner. You can avoid Photoshop altogether and just
use your scanner in preview mode. I use the scanner set to gray scale for
this.
Preview your Stouffer step wedge on your scanner. Place the selection box
for the area to be scanned inside a single step of the step wedge. Your
histogram of this preview will show a narrow sharp peak. Drag the input
adjustment sliders in the histogram preview to the leading and trailing
edges of this peak and record the values. Do this for each step.
Make a table of these values and reflection density for each corresponding
step of your step wedge.
To check a print for density preview it on your scanner in gray scale. Draw
a selection box over the area of the print you want to check. Bring up your
scanner preview histogram and use the table you've generated to determine
the density from the histogram K values.
Here is a table I use for reading transmission density of negatives:
http://img98.imageshack.us/img98/4999/scannerhistogramtotrans.jpg
I purchased an X-Rite 361 T densitometer to read density values of my camera
negatives for contrast adjustment in Pd printing before I figured the
scanner trick out. Comparing my camera negative density ranges read from the
X-Rite with the values determined from the scanner shows very good
correspondence in real-world use. I no longer bother with the X-Rite.
As an aside, I no longer read reflection density values from prints. Not
that density isn't useful, it is useful for comparing Dmax and highlight
clearing etc.
When measuring prints I use the Eye-One spectrophotometer to read LAB
Luminosity values. I use the Luminosity values to create the Photoshop
correction curves I apply to digital negatives using QTR and PDN since the
LAB L values are based on what the eye sees and I want my calibration to
match the eye.
One could use the same scanner preview method I described above to create a
table set up for LAB L if you wanted to.
I hope this helps. And I'm eager to hear more thoughts on using the scanner
this way.
-Harlan


On Sat, Dec 11, 2010 at 4:00 AM, <
alt-photo-process-list-request at lists.altphotolist.org> wrote:

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>   1. Re: Print Density Estimation From Scan (Eric Neilsen)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 10 Dec 2010 10:47:13 -0600
> From: "Eric Neilsen" <ejnphoto at sbcglobal.net>
> To: "'The alternative photographic processes mailing list'"
>        <alt-photo-process-list at lists.altphotolist.org>
> Subject: [alt-photo] Re: Print Density Estimation From Scan
> Message-ID: <013401cb9889$e748a3b0$b5d9eb10$@net>
> Content-Type: text/plain;       charset="us-ascii"
>
> Peter, What are you doing to lock down your base scan? What software and
> scanner are you using.
>
> Eric Neilsen
> 4101 Commerce Street, Suite 9
> Dallas, TX 75226
> 214-827-8301
>
> Let's Talk Photography
> www.ericneilsenphotography.com
> SKYPE ejprinter
>
> -----Original Message-----
> From: alt-photo-process-list-bounces at lists.altphotolist.org
> [mailto:alt-photo-process-list-bounces at lists.altphotolist.org] On Behalf
> Of
> Peter Friedrichsen
> Sent: Wednesday, December 08, 2010 6:18 PM
> To: The alternative photographic processes mailing list
> Subject: [alt-photo] Print Density Estimation From Scan
>
> Hi All,
>
> Just wondering if anyone is willing to comment on my method of
> estimating print density using a scanner and a Souffer reflective step
> wedge.
>
> The density estimate is based on comparing only the red binary values
> with those of a Stouffer step wedge of 0.15 density increments per
> step, and this is done in photoshop by browsing pixels. The red
> channel was chosen because of an iron stain that affects
> predominantly the green and blue channels. The image I used is
> labeled print 3a and can be found here:
>
> <http://picasaweb.google.com/pgfriedrichsen/FerricGumProcess#>
> http://picasaw
> eb.google.com/pgfriedrichsen/FerricGumProcess#
>
>
> The whitest area in the sky on the left side and about 1/2 way up
> gives me a value of 225. The first shades (lightest greys) above the
> left side of the roof give me 221, and the darkest shades just under
> the eaves troughs gives me 42. I have chosen to use the lightest grey
> values because there may be blown highlights and I don't want to
> include those in the density range. So, here is what I am using in summary:
>
> image density-----------pixel value-------step wedge # at this value
>
> lightest greys--------------221---------------less than 2 (use two as
> conservative)
> darkest greys---------------42---------------10
>
> difference density=10-2=8
>
> now I multiply 8.0 by the step density difference value of 0.15 per
> step: 8 x 0.15=1.2
>
> I get a density of 1.2.
>
> Anyone wish to comment on whether this is a sound technique for a
> reflective density measurement?
>
> Peter Friedrichsen
>
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> End of Alt-photo-process-list Digest, Vol 318, Issue 1
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