Re: Q: historical document preservation - ideal resolution

On Fri, 18 May 2007 05:29:52 GMT, Dennis Lee Bieber
<bieber.genealogy@xxxxxxxxxxxxx> wrote:

On Thu, 17 May 2007 20:26:29 -0800, Vampire Slayer
<Don'tHaveOne@xxxxxxxxxxx> declaimed the following in
soc.genealogy.misc:



I also usually scan documents at 300 dpi. Depending on what the
original looks like I'll do gray scale if it is black and white and
color if it is either in color or detail can be seen better in color.
Unless it's a pristine printed page I've found that black & white
scanning usually loses a lot of detail.

I normally scan photo's at 600 dpi. That seems to be the sweet spot
for me on quality versus file size. Normally the file size is between
5MB to 100MB depending on the physical size of the photo and whether
it's black & white or color. 600dpi enables me to zoom in and see
details that you can't normally see in the physical photo.

That's almost the opposite of what I might recommend. But then, I'm
looking at future output capability.

A pristine, high contrast, document -- B&W @ 600 (720)... Most
modern printers have a native resolution of 600 (720 for Epson), and
since B&W only requires paper and pure black dots, a scan at that
resolution is effectively a "photocopy".

Photos, if intended to be printed at "same size" get scanned at 300.
300PPI is considered optimal for high-end photoprinting (it's the
resolution used by NatGeo grade magazines). The convention is to have
the "source data" PPI at 2x the (halftone) LPI. On an old (non-photo)
CMYK printer, it requires a 16x16 dot "cell" to emulate full color
halftone. 150LPI (high grade photo halftone) means a printer putting out
2400DPI. Of course, modern photo printers with CcMmYK, or more, or with
variable size dots, can produce the color variations in less than a
16x16 cell.

So... photos scan at 300*scale (where scale is the intended output
magnification: say a 4x6" photo to be printed at 8x12" => 2x -- scan at
600, reset resolution [Photoshop, say] to 300PPI. An 8x10" to be printed
at 4x5", scan at 150DPI, reset resolution to 300PPI).

This applies to grey scale too, as producing full 256 levels of grey
(from white to black) also requires the 16x16 matrix (unless you have a
printer with half-black ink).

Long, rambling response ;-)

Some of the oldest photos don't even have 200dpi resolution.
Hand-coated glass plate negatives did not have consistent resolution
across the image. On the brighter side, there's little quality lost
in making a contact print - assuming the negative plate is emulsion
side down...

The photos of my wife's grandparents' 65th wedding anniversary were
taken with a 120 film camera (for the larger negatives) and 100 speed
film (for finer grain) and a big flash unit (to use a small aperture
for better depth of field). The 8x10 prints are very good and the
inscription on the cake in the corner of the pictures can be read
easily.
Unfortunately, the majority of the family pictures I've acquired were
made with inexpensive snapshot cameras (think plastic lenses) and
"cheapest available" or "most convenient" processing - usually "when
the roll is finished", so pictures with a processor-imprinted date of
May include a snowball fight in Georgia. These prints are notable for
their lack of resolution - and the number that are under/over exposed
and/or out of focus and/or show camera/subject movement. If the
negatives were available, I might be able to do more with the
improperly exposed images (I have a negative scanner). In the current
project of nearly 2000 pictures, there may be 30 negatives.
Unfortunately, the outlook seems to have been "If you have the
pictures, why do you need the negatives?" And the few remaining
negatives are, at best, only marginally useful because they were loose
in a drawer somewhere and have been nicked and scratched repeatedly.

I plan to scan the negatives of pictures I have taken, probably at
1800dpi (higher would be better, but we're talking about thousands of
35mm negatives), with a thumbnail positive image for the image
database (haven't decided on which one yet). The B&W negatives are
sleeved and in 3 ring binders with at least some information about
date/place/event. The color negatives are mostly in sleeves and
stored with the prints still waiting to be put in albums. Most of
these have date/place/event info with them.

And then there are the digital-only images. The early digital cameras
produced 300KB images on a par with tintypes (except that they're
color). Will anyone have historically important pictures taken wih a
cell phone? The current crop of 6MegaPixel and up cameras are capable
of better images than most of the snapshot cameras of the 20th century
- but how will those images be archived? Think how excited some
genealogist will be in 2095 to find a memory card that's marked
"Weathers Family Reunion - 2007" - and the letdown when there is no
way of retrieving the data... I've tried to maintain backward
compatibility as I've upgraded computers (usually by building my own).
The latest machine (with dual format, double layer DVD writer) will
also have a CD writer (not all DVD writers can produce a universally
readable CD) plus 3.5 and 5 1/4 drives.

John

.

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