Olympus-OM
[Top] [All Lists]

Re: [OM] dots and pixels per inch

Subject: Re: [OM] dots and pixels per inch
From: Jan Steinman <Jan@xxxxxxxxxxxxxx>
Date: Tue, 13 Jan 2009 17:32:22 -0800
> 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.

:::: "Are you all right?"
:::: "No, I'm 5/6ths right. My mother's maiden name is Wright, which  
makes me half wright, and my middle name is Wright, which is 1/3rd  
wright, for a total of 5/6ths."
:::: Jan Wright Steinman http://www.VeggieVanGogh.com


-- 
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/

<Prev in Thread] Current Thread [Next in Thread>
Sponsored by Tako
Impressum | Datenschutz