[filmscanners] Re: Understanding dpi

["Arthur Entlich" <artistic@telus.net>]

Most color film scanners use a CCD chip which has a series of three
lines across it each with a color filter over it, Red, Green or Blue,
which each are made up of a series of sensors.  (Nikon uses a slightly
different method, but I don't want to confuse things).

That line contains a specific number of sensors across it.  For
simplicity, let's assume a film frame is one inch across by 1.5" wide.
That would mean if the scanner claimed a 4000 dpi (really ppi or pixels
per inch) resolution, the image dimensions when a file was created would
be 6000 pixels by 4000 pixels.

The film or sensor stage is moved one pixel width per scan cycle until
6000 cycles (for a 1.5" "long" film frame) are achieved.

The image is actually projected onto the CCD sensor, so the sensor's
length might be larger or smaller than the film dimensions.

If the exact same sensor was used in a medium format film scanner, which
had, say a 2" wide film frame, that would be scanned at 2000 ppi, since
the same number of sensors would be reading information projected on it
from a film frame twice as wide.

I have simplified this process.  But yes, the file size grows 4x if the
scanner resolution is doubles, assuming the same bit depth capture is used.

Art

Bill0786@tconL.com wrote:

> I'm a bit perplexed at what the dpi means on a film scanner. Trying to
> compare apples to apples, will a 4000 dpi Brand X film scanner in theory
> produce a better quality image outputted than a 2000 dpi Brand X scanner,
> given that the output resolution is the same, say 1600 x 2400 pixels?
>
> Or does it simply mean the 4000 dpi scanner will output a much larger
> image than the 2000 dpi model?
>
> Thanks for clearing this up,
> Bill
>
>

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