RE: filmscanners: Scanner resolution (was: BWP seeks scanner)

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

> Austin Franklin wrote:
>
> "4000SPI (samples/inch) divided by 25.4 mm/inch = 157.48 samples/mm, which
> means it can always resolve a detail that is (157.48 samples/mm
> divided by 2
> for sampling frequency divided by 2 for line pairs) = 39 lp/mm is the
> MINIMUM resolution that a 4000SPI scanner can resolve.  That is for line
> pairs that are perfectly horizontal or vertical."
>
> Dividing by two *twice* in this calculation is not correct (we've had this
> conversation before).

Sure it is.  You even substantiate that below!

> The HIGHEST theoretical resolution that a 4000spi scanner can
> resolve - with
> line pairs that are perfectly horizontal or vertical - is 157.48/2 or 78.7
> lp/mm.

Read what you wrote first:  "The HIGHEST", ie, maximum.  Then, read what I
wrote "the MINIMUM".  Your "highest" (maximum) is NOT guaranteed resolvable.

Obviously the maximum the scanner can resolve IS the width of the sensor
element in a typical film scanner, which at 4000DPI, that is 4000/25.4/2 or
78.7.  Only if the 78.7 lp/mm line up perfectly with a sensor will you
resolve them with the highest contrast.

You have no contrast if the 78.7 lp/mm are off dead on by %50.  You will
always get the highest contrast resolving 39 lp/mm, which you are not
guaranteed getting with intermediate values above 39.