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filmscanners: Pixels per inch vs DPI

Ken wrote:
> see it relates mostly to size.  I'm still not entirely
> sure why high res scans look better on a screen only
> capable of displaying 72dpi.  I tried a slide at 2720
> and then 680 dpi, sized the two scans the same, and the
> 2720 looked far better, especially under high zooms.

The reason you're confused is that the term "dpi" is being used for several
things.  Scanner manufacturers talk about their scanners being (say) 2700
dpi.  What they mean is that it scans 2700 *pixels per inch*.  When you
display those pixels at 1:1 on a computer screen which has (say) 72 pixels
per inch, you'll only see a small part of the picture.  But if you print
the picture onto paper using a printer that prints 300 *dots* per inch,
your 2700 pixel per inch scan of a 35mm frame will end up around 10"x8".
 AFAICS that's why the scanner manufacturers picked 2700 ppi for their film
scanners - it roughly translates to a full A4 page print at 300dpi.

Even the printer manufacturers confuse the issue because Epson for instance
say their current printers print at 2880x720 "dots per inch", but the 
between pixels on the screen and colour on the page is more like 300 dots
per inch!  The printer can't print one dot of the colour you want because
the inks are only CMYK or CcMmYK, so it needs to use a pattern of dots to
make the colour in the file or on the screen.

I've been find of flamed for saying "screen resolution is 72dpi" but it's
a worthwhile rule of thumb.  Pick 100dpi if you prefer; it's certainly easier
to calculate with.  If you want a 2" x 2" image on the screen at 100dpi
you need 200x200 pixels.

The terminology is confusing, and the manufacturers aren't helping.  All
I've been trying to point out is the relationship between the various 
of different media and devices.

The rule of thumb I've seen most commonly expressed can be summarised as;

1. Scan at the maximum resolution of your film scanner (eg. 2710 ppi)
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 
of Vuescan to 300dpi.
3. If you want a screen image, you need to resize the scan (or portion of
the scan) so that it's about 100 ppi.  So a 3000x2000 pixel film scan would
have to be reduced by 10x to make a 300x200 pixel image suitable for a small
web jpeg.

I've used "pixels per inch" and "dots per inch" interchangeably and I'll
probably be flamed for it.  They aren't interchangeable in that you can
set a 100x100 *pixel* image to be any *dpi* you want.  That will just determine
the size of the image when printed.  I probably haven't been clear enough
in how I've written this, but hopefully you can see how the relationship
between pixels and size changes from device to device and medium to medium.

FWIW also, Epson photo printers print quite well at 240 ppi or even less,
you'll get a good image at 300ppi on most printers.  It's especially important
if you want to use a commercial printing company - they will insist on 300dpi


Rob Geraghty harper@wordweb.com


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