We are in agreement! :- )
Harvey Ferdschneider
partner, SKID Photography, NYC
Bill Fernandez wrote:
> Hi Harvey--
>
> "25mm" was a typo, sorry. I meant "35mm".
>
> I scanned a 35mm Kodak Q60 Ektachrome calibration target at 1200dpi
> on a flatbed scanner. Then I scanned a 4x5 inch Kodak Q60 Ektachrome
> calibration target on the same flatbed scanner, again at 1200dpi.
>
> The grey steps are supposed to be the same density on each target
> (Except for manufacturing tolerances).
>
> If in Photoshop you select a rectangular area covering, say, 90% of
> one of the grey steps and open the Histogram window it will display
> (among other things) a standard deviation number which represents how
> widely the pixels within the selection vary in tone from one another.
> This is a good way to see at what density levels the scanner starts
> getting noisy: just move the selection from one grey patch to the
> next and record the standard deviation of each step. At some point
> the standard deviation will start to rise, implying that those steps
> are noisier than the previous ones because there's more variation in
> the value of the pixels.
>
> Now when I did this on both of the above targets (remembering in each
> case to select the same percentage of each grey patch) I found that
> the standard deviations on the 35mm target's grey patches rose faster
> and higher than on the 4x5 inch target's.
>
> Initially I was perturbed until I realized that this was like taking
> a picture of the exact same scene on both 35mm and 4x5 film and
> making, say, 20 x 30 inch prints from both slides. The grain of the
> 35mm slide gets blown up five times as much as that of the 4x5 inch
> slide. I'm assuming that having big blobs of grain (35mm target)
> rather than a fine pattern thereof (4x5 target) has skewed the
> distribution of tones reflected in the standard deviation numbers:
> that to the histogram the grey patches look less uniformly grey on
> the scan of the 35mm target than the 4x5 target.
>
> --Bill
>
> At 11:27 PM -0400 23-10-01, SKID Photography wrote:
> >Bill,
> >Maybe then I'm not understanding you. If what you are saying is
> >true, then the 25mm sample is not the same
> >density as the larger format, because of the grain (or more
> >properly, the space between the grain).
> >
> >Harvey Ferdschneider
> >partner, SKID Photography, NYC
> >
> >Bill Fernandez wrote:-
> >
> >> So according to your summary of the discussion on the ColorSync list
> >> they decided that the artifacts being discussed were caused by
> >> scanner noise in dense areas. Certainly a well-known phenomenon. I
> >> on the other hand was referring to an instance where I caught myself
> >> erroneously attributing the effects of film grain to scanner noise.
> >> As far as I can tell we were talking about two different things.
> >>
> >> --Bill
> >>
> >> >At 1:39 PM -0400 23-10-01, SKID Photography wrote:
> >> >
> >> >The threads on the colorsync list...
> >> >The consensus of most was that the grain that was showing up in the
> >> >dense shadows from the greater
> >> enlargements (samples) was due to the problem of electronic noise,
> >> and not, in fact grain...
> >>
> >> >Bill Fernandez wrote:
> >> >...I couldn't understand why
> >> > > two targets of equal density on the same scanner could be so
> >> >> different; until I inspected both scans closely and realized that the
> >> >> 25mm target was hugely grainy, especially in the dense end of the
> > > >> greyscale. So what at first I thought was scanner noise turned out
> > > > > to be film grain
>
> --
>
> ======================================================================
> Bill Fernandez * User Interface Architect * Bill Fernandez Design
>
> (505) 346-3080 * bill_sub@billfernandez.com *
> ======================================================================
