[filmscanners] Re: PS sharpening

["Anthony Atkielski" <anthony@atkielski.com>]

Shunith writes:

> Could you kindly elaborate on that?

DPI has no meaning for digital images unless they are being printed on
paper.  Digital images themselves are dimensionless--they are not x inches
wide by y inches high, they contain only pixels.  Monitor displays are also
dimensionless; they contain only pixels as well.  So all that matters for
Web display is the number of pixels in an image; the DPI recorded in the
image file has no effect at all.

When printing images, _some_ programs will look at the DPI figure recorded
in the file and will attempt to scale the image for printing.  For example,
if the image is 1200 pixels wide, and the DPI is set to 300, some printing
programs will attempt to print the image 3 inches wide (1200/300); but this
is not at all universal, and many programs won't pay any attention to the
DPI.

Programs that do pay attention to DPI when printing include Quark XPress and
Photoshop.  But if you are not printing, DPI is completely irrelevant, and
you can set it to anything you want.

> Grateful if you could elaborate on that as
> well... specially the bit about a dpi (ppi)
> of 2700-4000.

If you set the DPI very high, and someone downloads your image from your
site and attempts to print it in a program that looks at the DPI number, the
result will be a postage-stamp-sized photo on paper.  The figures of 2700 or
4000 are typical for scanners; when you scan an image, typically the
scanning software sets the DPI to the scanning resolution of the scan, so a
2700-dpi scanner will set the DPI to 2700.  If you then print the image in a
program that looks at the DPI number, it will be printed at exactly the size
of the original slide or transparency, that is, 36x24 mm, on paper.  This is
so small that it is unusable for most purposes, and since many people don't
know how to change DPI, if you set the DPI high in your images on your site,
it will discourage many people who might try to download and print the
images--they'll see the tiny image on paper and give up.  (A few, of course,
will know what is wrong and will just change the DPI and print a larger
image.)  It's just one more minor way of discouraging image theft.

I usually leave images on my site set to the DPI of the scans, so they are
always at 2700 or 4000 dpi.

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