John,
This is one of the most misunderstood aspects of digital photography,
and the amount of confusion out there is actually very simple. An
image, whether scanned or captured digitally, has a size, measured in
pixels. For example, 4000x3000 pixels (from a 12 MP camera, to use
round numbers). You can print such an image in various sizes, each of
which will have a different resolution. For example, if you print it
as 10x8 inches, the resultion will be 400 pixels per inch (measured on
the long side).
When your photo lab asks for images with at least 300 pixels per inch,
what they mean is that you should provide an image of sufficient pixel
dimensions to yield a resolution of 300 ppi. In the above example, if
you ask them to make a 10x8 inch print or smaller, you are fine. If
you ask them to make a print that is 2 feet on the long side, then
your resolution will be only 4000/24=167 ppi, and the print will not
have sufficient definition according to their standards.
So, in a nutshell, it is misleading of an image having a resolution of
x. It has pixel dimensions of x by z, and the resolution dependes
entirely on the size of the print you wish to make from it.
Nathan
Nathan Wajsman
Alicante, Spain
http://www.frozenlight.eu
http://www.greatpix.eu
http://www.nathanfoto.com
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On Jan 13, 2009, at 3:56 PM, John Hudson wrote:
> To quote www.NetLingo.com
>
> "Resolution
> "A common computing term, it refers to several things. On a computer
> monitor, it is the number of pixels (horizontally) and lines
> (vertically) on
> the screen. For printers, resolution is a measurement expressed in
> dpi that
> describes the sharpness of a printed image. With sound boards, it is
> the
> number of bits used to encode sounds. And in business, it describes
> some
> form of mutual agreement."
>
>
>
> My printer will work at the following print resolutions; draft,
> 360d[ots]
> per inch through various steps to 2880 d[ots] per inch
>
> Whenever I get prints done out of the house the photo lab asks for
> images at
> no less than 300 p[ixels] per inch.
>
> Is anyone able to provide a clear explanation of the relationship
> between
> pixels per inch and dots per inch ?
>
> Does an increase in pixels per inch generate an increase in dots per
> inch or
> are the two mutally exclusive ?
>
> If a printer's technology can only deliver a maximum or so many dots
> per
> inch is there a formula to show that an image resolution beyond so
> many
> pixels an inch is un-necessary ?
>
> If this question has already been flogged to death I apologise for
> asking
> again.
>
> John Hudson
>
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> --
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