Just to muddy the waters further, when you get into dpi with printers, it opens 
up a whole 'nother can of worms...at least as I understand from researching the 
subject on the net.

With regard to printing, dpi and ppi are two seperate things.  A printer might 
have a certain dpi rating, but that is NOT (again, if I understand correctly) 
the same as ppi. Why, because there may be more than one dot per pixel.  You 
can have a single pixel that may be rendered by the printer with a grid of very 
small dots...and that pixel will still be at 300 ppi on the page, but that 300 
may be at a much higher dpi, depending on the resolution of the printer.  
Capische?  In other words you'd have 1200 dpi, rendering 300 ppi, or somesuch.

For printing purposes, what I came across was that anything up to say 5x7 
inches, you want to print at 300 pixels per inch.  As the print size gets 
larger, this value can go down.  For instance, an 11 x 14 print could probably 
go at something like 250 pixels per inch, or even 200.  A much larger print, 
like a 16 x 20, maybe 150 pixels per inch.

Why?  Because smaller images are usually viewed much closer than larger ones.  
So smaller images need more resolution.  But as you get farther away from the 
image, less detail is needed for an "acceptable" image.  It is also my 
understanding that in some respects digital imaging is more forgiving than 
film, since it tends to have a smoother perceived "graininess" than film.  One 
of my photography teachers didn't like us blowing 35mm up larger than 5x7, 
since he felt that image quality tended to suffer.

I used to have a medium format camera (6X7), and had some 5x7's printed off of 
it.  I was blown away by how incredibly sharp they were.  However, nowadays, 
some of your high end digital cameras have just as good of perceived 
resolution.  It is my understanding that 6 megapixels is the bare minimum 
needed to equal the resolution of 35mm.

However, there is also the actual physical size of the digital image sensor to 
consider.  Smaller sensor with lots of pixels will be "noisier" (i.e. granier) 
than a physically larger sensor with fewer megapixels.  The larger the sensor, 
the better, as it makes for smoother noise.  If you get a chance to see 
anything shot with a Canon 5D, you will see what I am talking about.  The 5D 
has a full 35mm size sensor and the output from this camera is phenomenal.  I 
am no expert, but it wouldn't surprise me to find that this camera has quality 
as good as older film based medium format cameras, if not better.

As for scanners, they have a certain "native" or "optical" resolution, at which 
they are best used.  The software that comes with them can "interpolate" and 
create higher resolution images, but it isn't done by actually scanning 
something.  As is mentioned earlier in this thread, it uses some fancy math to 
fill in the extra pixels not actually scanned.  The results can be blurry.  You 
get the same effect with scaling an image progressively in Gimp.  You take an 
image, scale it up by 10%, take the result and scale THAT up by 10%, and so on. 
 The result will be fairly smooth, i.e., not pixelated, but will be blurry, 
since there is no actual information added to what you started with, instead 
the computer "guesses" at what would have been there at a higher resolution.

--- On Tue, 4/7/09, norman <nor...@littletank.org> wrote:

> From: norman <nor...@littletank.org>
> Subject: Re: [Gimp-user] pixels to dpi
> To: gimp-user@lists.XCF.Berkeley.EDU
> Date: Tuesday, April 7, 2009, 12:16 PM
> < great big snip >
> This topic has certainly brought lots of reaction and my
> thanks to all.
> My intention is to buy a new scanner which I want to use
> for scanning
> photographs prior to carrying out restoration work. I use
> Ubuntu 8.10,
> GIMP and XSane.
> The book I have says that I should look for a scanner that
> captures at
> least 10 bits of data and has an optical resolution of 600
> pixels per
> inch. I don't have lots of money and this interest in
> restoration is
> just as a hobby and not a business. So, could some kind
> person guide me
> in the right direction, please.
> Norman 
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