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



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). 

Nyquist tells us that we must divide the sampling frequency in two to
calculate the maximum spatial frequency (to allow for one bright line, then
one dark line). By dividing once "for sampling frequency" and once "for line
pairs" you're doing the *same correction twice*.

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. Of course various optical and CCD effects can degrade this maximum,
which is how we get to intermediate figures such as McNamara's 60lp/mm.

-- 
David Morton
dmorton@journalist.co.uk

"The more opinions you have, the less you see." -- Wim Wenders.







 




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