On Fri, 07 Dec 2001 23:20:32 -0500 SKID Photography (skid@bway.net)
wrote:
> If you claim that
> 'scans', are not of the grain of the film, I don't understand where the
> scanner is getting it's information
> from.
For once I agree with Austin :-)
The scanner sees only luminance and colour, not image detail or grain
boundaries. It is essentially blind to all contrast transitions which have
a higher frequency than half the CCD pitch.
With film, image detail *always* has a much lower frequency than the grain
which constitutes the image. Grain acts as an irregular halftoning, and
although the implicit resolution capability is empirical rather than
calculable, the Nyquist limit still holds for film. That is why fast film
can record detail less well than slow films.
It is quite possible to scan and transpose this image detail without
recording grain boundary information.
In fact the CCD acts as a low-pass filter, which adds some distortions to
frequencies close to the filter cut-off point (as aliasing).
If a grain image at the CCD is smaller than a pixel, the pixel averages
the values it is presented with and an aliased representation is the
result.
If a grain image is larger than a pixel, this still happens at pixels
where there is only a partial overlap - the edge of the grain will produce
an aliased pixel value. Only fully-overlapped pixels will be free of
aliasing errors.
Whatever you see in a scan as grain is an aggregated, aliased
representation of grain mapped to the physical grid of the pixels. Only if
pixels are very much smaller than grains can this be an approximately
accurate resolution of the grain. Existing CCD scanners just don't get
near that. In fact I have seen drumscanned images of 4,000ppi, 8,000ppi
and 12,000ppi from ISO100 Ektachrome where each successive iteration
produced marginally more fine image detail yet significantly more
accurately rendered grain.
Even if you had perfect optics and could scan at 100,000ppi, you'd still
get aliased, fuzzy pixels instead of grain boundaries. They'd just be
very, very small pixels.
The same is true of actual image boundaries, except being of a lower
spatial frequency, the distortion is a lower percentage. Losses are
inevitable in getting stuff from the analogue to digital domain.
Scanning is like duping. Dupe onto a film which is coarser than the
orginal at 1:1 and you will lose image detail and sharpness. Dupe at 10x
and you will preserve most of the sharpness, but the original grain will
be compounded by the copy film grain. Use a fine-grain film instead, and
everything is less worse. However losses are inescapable, and
acceptability a judgement call.
So does any of this matter? That depends purely on output size and
technology, and viewing distance, plus how good your eyes are :)
Regards
Tony Sleep
- Online portfolio & exhibit; + film scanner
info & comparisons
