[filmscanners] RE: Film resolution - was: Re: 3 year wait

["Austin Franklin" <darkroom@ix.netcom.com>]

Hi Mike,

> > But, one thing that is VERY important is there is a difference between
> > sampling sine waves and square waves.  These test patterns are
> square waves.
> > Though at Nyquist, you guarantee detecting the frequency, you do not
> > guarantee full amplitude.
> >
> > To sample square waves, and GUARANTEE getting at least ONE "sensor" that
> > contains full amplitude, you MUST sample at 4x f (or 2x the
> line width, NOT
> > line pair width)...and in the case of an image, f really
> doesn't matter, but
> > the thickness of the line (which would be 2f).
>
> When sampling a square wave at the nyquist frequency, the square wave can
> be perfectly reconstructed from the resulting samples.

As I've pointed out, at Nyquist, when the line is not entirely "seen" by ONE
sensors, two sensors will only see a reduced part of the line, and therefore
the detected amplitude of the line will be decreased.  That is NOT
"perfectly reconstructed", as you say, since you are not reconstructing the
amplitude accurately.  The ONLY way to guarantee acquiring full amplitude is
for the sensor spacing to be at least 4f, which guarantees that a line of
width f will be fully detected by at least one sensor.

Try drawing this out on paper, or do something to visualize it...or heck,
design a scanner ;-)

Austin

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