Re: Resolution of graphics for printed docs

Subject: Re: Resolution of graphics for printed docs
From: "Chris Christner" <cchris -at- toptechwriter -dot- us>
To: "TECHWR-L" <techwr-l -at- lists -dot- techwr-l -dot- com>
Date: Wed, 5 Oct 2005 10:42:07 -0600

Hi Bill,

Bill Swallow wrote:
> > - Pixel dimensions: 504 pixels wide by 315 pixels tall
> > - Print size: 7 inches wide by 4.375 inches high at a resolution of 72
> > pixels per inch (ppi).
> First fuzzy issue... a screen capture does not have a dimension in
> inches. Not at all. A graphic does not either. Whatever application
> you're using to work with the image is telling you an output size
> based on the dpi value you set. That's it.

A 72 ppi image means there's 72 pixels per inch. So with a 504-pixel-wide
image, if you divide 504 by 72, you get 7 inches, which is how Photoshop
comes up with that value. Now here's the important bit: Photoshop calls
this the "Print size" because that's the size it will be output from a
printer. You don't have to believe me, just print out a web page and see
how it fills the page (unless you've set a reduction percentage).

> > Now, if you wanted to include the capture in a manual for printing, 7 x
> > 4.3 inches would be too big and 72 ppi printing at 300 or 600 dpi would
> > look too crude, so what to do?
> Well, crude how? And, again, electronic images do not have canned
> print size dimensions.

Crude as in a printed 72-ppi image looks huge, and photos printed at that
resolution have compression artifacts that make the images look blurry.

> > Well, you could simply change the resolution to 150 ppi, which would give
> > decent results when printed, but the file size would double and you would
> > still have to reduce the image's size in Quark or InDesign so it would fi=
> t
> > on a page.
> Well, not really. You can change the ppi values all you want and the
> image file size will not change. Why? Because this is a number that is
> passed to an output device, telling it how to render the graphical
> info in the image file. You're dealing with numeric info in the image
> file's metadata.

If you double the ppi with resampling turned on in Photoshop, the file
size doubles. If you do the same with no resampling the file size stays
the same, but the image's print size is reduced by 50%.

> > A better answer would be to reduce the image's print size while at the
> > same time increasing its resolution. That way the file size remains the
> > same while the resolution is increased to print quality. So we'd reduce
> > the print size by 50% to 3.5 W x 2.1875 H and double the resolution to 15=
> 0
> > ppi (it should be 144, but 150's close enough and works better with
> > printers).
> This makes no sense. Most printers don't care about the ppi you set
> unless it's outside their output range (in which case it'll do the
> best it can and leave it at that). Again, print size and resolution
> are joined at the hip, and only the resolution matters, as print size
> is arbitrary based on resolution.

That's enough, I think the rest of what I wrote in that post stands well



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