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     áòèé÷ :: Filmscanners
Filmscanners mailing list archive (filmscanners@halftone.co.uk)

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[filmscanners] RE: Understanding dpi



Art,

> 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.

You are correct for a magnification of 1:1, but not all scanners are 1:1.

> 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.

As a note, some MF scanners do scan 1:1, for all film formats.

I believe *most* 35mm only scanners use a 1" wide sensor.  Most MF scanners
use a 2.25" (6cm) wide sensor.  The spec sheet for a particular scanner
should show that information.  Having multiple magnifications requires a
couple of moving stages, and it's typically more economical (these days) and
accurate to simply fix these stages, and scan everything at the same
resolution.

The quick version of SPI/PPI/DPI is scans are done in samples per inch, and
the resultant image data is pixels.  Pixels per inch get sent to the
printer, which converts the pixels to dots, and prints dots per inch...
This isn't a correction to what you said, just, I believe, a simplification.

Regards,

Austin

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