> From: John Hudson <OM4T@xxxxxxxxxxx>
>
> 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 ?
Your printer can lay down any of its individual inks in a grid that is
(at most) 2,880 individual ink drops per inch.
It takes much more than one ink drop to make up the ~16 million
possible colours in a pixel.
The human eye can resolve about 300 individual pixels per inch from
the closest focusing (under ~40 years old) distance of about 6" away.
So, the service bureau wants 300 pixels per inch so that viewers won't
see the pixels. This requires more than 300 distinctly-coloured ink
drops (normally called "process colours" or "additive secondary
colours") in the same area, in order to make up the full-colour pixel.
That's the simple explanation.
> Does an increase in pixels per inch generate an increase in dots per
> inch or
> are the two mutally exclusive ?
The hole thing gets complicated by different "dithering" schemes used
to translate an arbitrarily-coloured pixel into a pattern of
distinctly-coloured ink drops. The printer driver or "raster image
processor" (RIP) maps one to the other.
Generally, an increase in ppi beyond 300 is barely discernible, and
printer and RIP design takes that into account. So if you tell it to
send more than about 300ppi to the printer, you generally won't be
able to see any improvement.
> 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 ?
Generally, there's no good reason to send more than 300ppi to the
printer -- and that's for "fine art" use.
For "scapbook" use -- something that isn't going to be seen at less
than arms' length viewing distance -- there's generally no good reason
to send more than 150ppi to the printer.
Billboards, designed to be viewed at several hundred feet, are printed
at 8-15 ppi!
It's the angular resolution at normal viewing distance that is
important. Yes, there are formulas for this sort of thing.
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