P3D Re: RE:Scanners

["Greg Wageman" <gjw@xxxxxxxxxx>]

From: Perry, Duane <PerryD@xxxxxxxxxxxx>


>I can see zillions of hours tied
>up in digitizing my old photos, my fear is I will complete this
monumental
>task and someone will come out with new scanner that renders everything
I
>did to the "old technology" pile of things I will never use again.

Why should you fear that?  Assuming your scanner does a reasonably
accurate job, and you scan at a reasonably high resolution, why should
newer technology render that data obsolete?  Pixels are pixels, after
all.  If the scanned images look true to the originals, a newer scanner
will not magically make them better somehow.

What is inevitable is that the resolution of scanners, printers and
displays will continue to increase.  Images scanned at 640x480 pixels
once filled an entire display.  Now those are good enough for web
graphics only.  Scan at the highest resolution you can afford to store.
The good news is that the size of harddisks continues to increase while
the cost remains relatively constant.  And as long as you keep the
originals safely, you can always re-scan at higher resolutions when that
finally becomes necessary and/or practical.

>I would like for people to be able to print out 8x10's that look sharp,
is 1600x1200
>about right?  How large are grey-scale files at that resolution?


The first is a complicated question, because it depends on the
resolution of the output device.  The important thing is that you don't
want to have to enlarge the digital image when printing, because this
will make it "pixel-ly".  Ideally, you match the scanned resolution to
the output device for the desired final size, so you're neither storing
data you can't use, nor enlarging, but it's also OK in terms of image
quality if you have to reduce it somewhat to fit the printer.

For example, if you scan a 1-inch by 1.5-inch slide or negative at 600
dpi, and print it on a 300 dpi dye-sub or other "true color" printer, it
will come out 600/300 or 2x larger, i.e. 2x3 inches.  At 1200 dpi
scanned resolution, it will print at 4 times the original size, or 4 by
6 inches.  If the output device cannot do 'true color' then the
calculations become more difficult.  You will need to know the effective
resolution after the digital halftoning algorithm has been applied,
which can be 1/3, 1/4 or an even smaller fraction of the resolution in
dots.

As to your second question, again the answer is "it depends".  How many
shades of gray are we talking about, and what compression factor?  1600
times 1200 pixels is 1,920,000 pixels, but then you have to multiply by
the color depth.  Sixteen bits-per-pixel will store 2^16 (65536) levels,
and require 3,840,000 bytes to store (not counting overhead like headers
and other ancillary data), but that could be compressed to as little as
1,280,000 bytes (at 3:1).  In many cases, the type of compression and
compression ratio are determined by the file format (e.g. GIF, PNG).
Some, like JPEG, let you trade off file size for image quality.  At
roughly a megabyte per image, you could fit over 600 such images on a
single CDROM.