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filmscanners: RE: filmscanners: Pixels per inch vs DPI

Austin wrote:
> Certainly a prolific problem.  I prefer to say SPI as it
> relates to the scanner, PPI is what you output TO the
> printer driver, and DPI is what the printer prints.

*I* know you mean samples per inch, pixels per inch and dots per inch, but
a newbie will find all the terminology confusing - especially when their
scanner and printer manufacturers use "dpi" exclusively in their interfaces.

> Yes it is, and posting someone?s ?opinion? isn?t
> a ?flame? at all, not even ?kind of?...

*shrug* Of course I don't have a problem with anyone expressing their opinion.
 Theirs is just as valid (or maybe moreso) than mine.

>> 1. Scan at the maximum resolution of your film
>> scanner (eg. 2710 ppi)
> Optical resolution...agreed.  Some scanners list
> interpolated resolutions as their maximum resolution...

OK, I didn't mention that caveat since most *film* scanners seem to work
with optical resolution.  My Nikon interface won't let me set an interpolated
resolution.  Flatbeds are famous for claiming to be "9600dpi" when their
optical resolution might only be about 300dpi.

>> 2. If you want to print the picture, the maximum size you can
>> print is limited to the number of pixels expressed at 300 ppi.
>>  I always set the output resolution of Vuescan to 300dpi.
>I have no idea what you mean by that...would you please elaborate?

OK.  I don't have any files to work with here so I'll work with some theoretical
numbers.  Suppose you scan a 35mm frame at 2700dpi (spi if you prefer :)
and get a file containing 3000x2000 pixels.  If you want to print the file
at 300dpi (or 300 pixels per inch) the maximum print size will be 10" x
6.7".  I've just divided the number of pixels by the required number of
pixels per inch.  In practice the proportions of a 35mm frame should give
you about 10"x8".  This is just a general rule of thumb; translate the pixels
to an equivalent 300 pixels per inch and that is the "best" print size you'll
get from the digital image.

1. If you want to resample the original scan, you can print at any size
you like, but there will be artifacts from the resampling process.
2. As I mentioned earlier, some printers give quite good results at lower
dpi.  Epsons seem to work quite well at 240dpi because of the integer 
with the 1440dpi native dot size.

Does that make more sense?


Rob Geraghty harper@wordweb.com


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