Picture size

"P & H Macguire" wrote in news:yFe1i.14160$8E.416
@newsfe5-win.ntli.net:

I am in the process of scanning some old slides of 60s to 90s and
wondered what the optimum size should be for posting on this N/G.
They will be scanned at about 300dpi.

Regards

PJM


Okay.

To be technical, DPI is a printing resolution, and you cannot scan
at that. This is nit-picky, because the actual term for scanning
resolution is PPI (pixels per inch), and most people use them
interchangeably, but there is a difference.

For scanning slides, start at max resolution and try working
backwards on the sharpest slides, watching the differences carefully at
100% in your image editing program. This will largely appear to be
blotches of indistinct color, but you can see whether the higher
resolutions make any difference to your slides. Super sharp slides may
benefit from scanning at 2400 PPI or higher, but film grain, lens
quality, and steadiness of the photographer all play a part, and pics of
lesser quality may not show any improvement between 1200 PPI and 2400 PPI
because, quite simply, the resolution isn't there in the film.

For scanning prints, you typically will not pull up much, if any,
detail beyond 300 PPI. Magazine prints can typically go lower, but
scanning at a higher res with the page at a slight angle helps correct
for screening (moire) patterns, and the pic can be straightened after
scanning in your editing program.

I have never discerned any difference between saving in TIFF
(lossless) format and highest quality JPEG, except for the tremendous
savings in file size - this, mind you, is for the original scan. Archive
off the original scans and back them up - you never know when a slide
will get damaged or disappear. And always work on a copy. There's nothing
more frustrating than hitting "Save" instead of "Save As..." and
overwriting your original scan.

As for display resolution, you can ignore DPI entirely - it means
absolutely nothing to the monitor or display. My vote goes for 1024x768
pixels or smaller, because I'm one of those miserly people who maintains
that as a monitor resolution. Larger just means scrolling, and that
detracts from the affect of the shot, especially if you're the type to
frame a shot carefully.

JPEG compression seems to work just fine at between 50% and 80%
quality (100 being full quality, not compressed) - it depends on, not the
detail of the shot, but the gradients, which is where jpeg compression
has its first affects. If you see your skies becoming blocky in places,
increase quality.

Sharpening should be done sparingly, if at all. If you see halos or
fringing occurring along areas of high contrast, you're sharpening too
much.

More info than you asked for:

DPI is considered a printing resolution, but most printers nowadays
ignore it entirely and simply interpolate what is needed from the final
print size you indicate. Even the home inkjets will print far more than
300 dots per inch, but this is because they have to make a 16 million
color gamut from 6 ink colors or less, and have to layer in multiple tiny
ink dots to give the impression of a field of clear Prussian Blue.

About the only place I've seen DPI make any difference whatsoever
has been when you're laying text in over the image (like a copyright
mark). Photoshop, at least, judges font size on the DPI resolution, so
pick one and stay with it. It can be 1 DPI if you like, and the file info
may consider your pic to be a thousand inches wide, but it'll still
display one pixel per pixel on a monitor at 100%. 300 and 72 DPI are the
defaults, and either one works fine. Information like that is ignored by
both browsers and monitors, which only work from the pixel dimensions.

And in fact, even for web pages, the photo displays at the pixel
dimensions specified in the html, which may not be the pixel dimensions
of the jpeg file. Lots of novice web designers cause huge page loading
delays because they take a jpeg way too large for the web and "size it"
in the html. All it means is a large file takes time loading just to
display at a res that it could have been sized to in the first place.


- Al.

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