"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.
At 11:27 PM -0400 23-10-01, SKID Photography wrote:
>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).
>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.
>> >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
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