From: HI100@xxxxxxx
chris@xxxxxxxxxxxxxx writes:
<< 100 lines per millimetre is 200 pixels per millimetre.
200px times 25.4mm is 5080 ppi.
cjb. >>
[Lot of great stuff from Tim deleted...]
... To sample and reconstruct
a grating of lines (a square wave) and produce sharp high contrast
edges (not turn them into a lower contrast sine wave) you need a whole
lot more.
I think we're comparing apples with oranges here. Correct me if I'm
wrong, but "resolving line per millimeter" means that one can discern
lines, not that they have sharp edges. In that respect, the result of
a visual test of lpmm is not unlike scanning near the Nyquist limit
-- fuzzy things one can just discern as lines.
Also, the sensor cell size performs inherent low-pass filtering of
the result. For aliasing artifacts to be visible, the sampled area
would have to approach a point. This is why scans can all stand a
little careful sharpening.
It would be better if some totally objective test (such as MTF?)
could be used for specifying scanners, but for now, it looks like the
marketing departments are in charge. )-:
I'm not disagreeing with your technical knowledge, Tim -- just
wondering if it really matters?
(BTW: my rule-of-thumb is ten times the base frequency to reproduce a
square wave. Thus a 100 lpmm grid requires 50,800 samples per inch to
reproduce with sharp, square edges.)
No wonder subjective evaluation of results is important.
Here, here! Buy a scanner based only on specs to your own peril! I've
done pretty good by careful research, particularly of reviews of
multiple models at the same time by magazines that actually have test
labs. (Many reviews are simply reviews of the manufacturer's press
releases.)-:
: Jan Steinman <mailto:Jan@xxxxxxxxxxxxxx>
: Bytesmiths <http://www.bytesmiths.com>
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