U of S | Mailing List Archive | alt-photo-process-l | Re: Measuring DMAX

Re: Measuring DMAX


Picture Window Pro (PWP) uses the following equation to tell us the density
of screen image values

-log10((pv/(2^bits - 1))^gamma)

pv => pixel value
bits => bit resolution of the image (8 or 16 per channel)
gamma => monitor gamma value (most probably 2.2 on PC and 1.8 on Macs)

In the case of scanned image, while in VueScan or some other scanning
software you may want check it out with a gamma of 1.

This may be enough to give you a relative density value one can use to
compare various process but you most not make any adjustment to the image
before making the calculations otherwise the pixel values will be scaled all
in the same range [0..255] and you wont be able to compare anything.


----- Original Message -----
From: "Kees Brandenburg" <ctb@zeelandnet.nl>
To: <alt-photo-process-l@usask.ca>
Sent: Tuesday, January 15, 2008 3:46 AM
Subject: Re: Measuring DMAX

> Hi Marek,
> Vuescan <http://www.hamrick.com> can do that. First check the 'Enable
> Density Display' option in preferences. Then point your mouse to a
> location in the scan/preview window and hold the control key. Vuescan
> displays the log density readings of the raw scan. These readings are
> not altered by your scanner settings or profile.
> Kees
> On 14 jan 2008, at 22:02, Marek Matusz wrote:
> > Is there a way to measure the DMAX of a print with a scanner /
> > Photoshop? I do not have a densitometer at hand. I vaguely remember
> > a thread a while back, but could not find it.
> > Marek