Re: Help me understand (dpi, scaling, resolution)

The best place to learn about scanning is do a lot of reading on:
http://scantips.com/

Start reading at "START".

--
CSM1
http://www.carlmcmillan.com
--
"Barry Watzman" <WatzmanNOSPAM@xxxxxxxxxx> wrote in message
news:4c36f$4941c5c4$11126@xxxxxxxxxxxxxxxxxxxx
Much of this is confusing terminology and also depends on how your
software works.

On SOME software (example: Nikon Scan on a Nikon film scanner) you just
set a dpi, and it's based on the "real" dimensions of the image. For
35mm, 2,700 dpi gives about a 10 megapixel image.

But on some other software (example: an HP flat bed document scanner with
a transparency adapter for slides), you have to establish "output
dimensions" (e.g., 4"x6" or 8"x10") and you set the dpi relative to the
OUTPUT image size, so if you set 300 dpi on an 8x10 output size (and, in
this case, that is how you set it), you will scan 2400x3000 pixels (over
the same 35mm slide).

A couple of thoughts:

Somewhere around 6 to 10 megapixels spread over the 35mm film (slide or
negative) image, you really are scanning all the detail that the TYPICAL
35mm film image has.

Another consideration, however, is "real" vs. "interpolated" pixels. If
at all possible, you want to use a resolution that will give an image
consisting entirely of "real" pixels (interpolated pixels are pixels that
exist where there is no actual image sensor element. They are
"calculated" by looking at the nearest real pixels on either side of the
interpolated pixel position and assuming a linear transition between those
points. Fine, but the actual image never bothered to study such
mathematics and that may not represent what was really in the image).
Again, you have to know your hardware and your software.

In truth, I like the Nikon approach much better ... set the actual DPI to
be used by the actual scan based only on the actual size of the 35mm image
[the image sensors in various Nikon scanners are 2,700, 2,900 and 4,000
dpi .... anything else is interpolated]. The output size is artificial
(and Nikon just uses the actual output size ... about 1" x 1.4"), and if
you want to rescale the image later, you can do that in any image editing
software (I am talking here about a rescaling in which the dpi and
dimensions are both changed such that the total number of pixels does not
change (at all) and every pixel remains an actual "real" pixel).

Re: "Or am i completely off base here and should be scanning at the max
dpi?" .... well, in most cases, the "maximum dpi" is based on interpolated
(sometimes HIGHLY (e.g. ridiculously) interpolated) pixels. Never scan at
a resolution so high that you are at more than twice the "real" resolution
of the scanning hardware (and even that is arguable, many would say to
always and only scan at exactly the maximum hardware resolution, unless
scanning at exactly half of that will meet all of your needs).

Unrelated to your question, in my mind the only acceptable hardware for
archiving 35mm images to digital are the Nikon film scanners and some
models of Epson scanners. I would not use anything else.


There is no substitute for understanding your hardware and software

barnes_jude@xxxxxxxxx wrote:
I am scanning 35MM slides.

I have read up on the internet on this a bit and mostly see
recommendations to scan at highest dpi the scanner will allow when
scanning slides. However I have a colleague in the graphics dept who
says better to scan at 300dpi and "scale" it appropriately.

Just a quick example: What do I gain by scanning at 600 dpi scaled to
8 x 10, as opposed to 300dpi scaled to 8x10, when I look at the image,
I do not see much difference in detail, even when I re-sample at both
settings into a zoomed area on the slide which possess detail (such as
spokes on a bike wheel).

Or am i completely off base here and should be scanning at the max
dpi? I noticed when I do this, the scanner software does not allow
scaling. I also notice that I achieve simlar file sizes when I scale
up a lower dpi scan to a larger target size, when compared to a larger
dpi set to a smaller target size. Are they apples to apples or are
the images actually significantly different and I just can't see it on
my monitor and 8x10 test printouts I've been making?

Thanks so much in advance, I really want to get this archiving
underway for the family!


** Posted from http://www.teranews.com **


.

Relevant Pages

  • Re: Archival Scanning
    ... I have heard that 300 dpi is the desired output but the ... The only suggestion I would make would be to reconsider the resolution for your negatives, depending on how particular you want to be and the equipment available to you. ... Negatives will contain more intrinsic information than any of the other image sources available and it is a reasonable assumption that the scanner itself will limit just how much of that information you can capture. ... Also, remember that the scanner's quoted resolution is just its sample density, the smallest step between image pixels, and it may not be able to optically resolve 1/2400th of an inch, possible as little as half of that in practice, depending on the scanner. ...
    (comp.periphs.scanners)
  • Re: How big do you scan a 35 mm original?
    ... Have I been missing some quality by not scanning at a higher resolution on my inexpensive desktop scanner? ... The quality of the raw scans is shabby compared to what a high-end drum scanner can do, and there were NO Photoshop tweaks applied, other than saving both as JPEGs in the size you see, down-sampled to 1080 pixels wide, with the same amount of compression. ...
    (rec.photo.digital)
  • Re: Help me understand (dpi, scaling, resolution)
    ... What do I gain by scanning at 600 dpi scaled to ... I noticed when I do this, the scanner software does not allow ... you scan at the optical resolution of the ...
    (comp.periphs.scanners)
  • Re: Help me understand (dpi, scaling, resolution)
    ... What do I gain by scanning at 600 dpi scaled to ... I noticed when I do this, the scanner software does not allow ... Then, if you want to re-sample, do it with a quality program in your computer. ...
    (comp.periphs.scanners)
  • Re: Copying Images To CD Confusion
    ... The terms ppi and dpi are often ... you would not normally scan at such a low resolution as 300 ... your scanner, either -- it would probably create humungously large files. ... Not usually during scanning. ...
    (rec.photo.digital)
Loading