Re: filmscanners: RE: filmscanners: Re: Hello, thanks, and more.

[Arthur Entlich <artistic@ampsc.com>]

OK, now I have a better understanding of your question.

The big problem most people have is seeing images are composed of two
things, dimension (the size in inches, for instance) and then resolution
(the number of pixels that make up each inch.)

This makes things more complex than necessary.  There are no dimensions
to an image within the computer.  There are only number of pixels.

As the number of pixels gets spread out (lower resolution) the image
dimensions grow, and vice versa.

So, a 100 dpi scan of a 4 x 6 print, would have the pixel dimensions of
400 x 600 pixels.  Shown on a screen which is set to a resolution of 100
dpi, the file would produce an image 4 x6".  BUT, if the screen
resolution was (hypothetically) only 50 dpi, the same file would show up
as a screen image 8" x 12".  On the other hand, a screen set up with 200
dpi resolution, would make the same file appear as an image only 2" x 3".

NOw, you mention a flatbed scanning a standard 4 x 6" snapshot, which
you wish to, let's say, fill the screen with.  And let's say your screen
has the pixel dimensions of 800 x 600.  That means you need to scan the
print in a manner that you would get approximately 800 pixels in the
long direction, and 600 in the other.

To accomplish this with a 4 x 6" print, you need to scan at 6 divided
into 800 or 4 divided into 600.  The result is 133.333 pixels per inch
in the long direction and 150 pixels per inch in the 4" direction.  So,
if you really want this image to fit the whole 800 x 600 pixel screen,
you'd have to scan at 150 dpi, making an scanned image 900 x 600 pixels,
and then crop off a bit from the sides (50 pixels each).  Or, in theory,
you could scan at 133 pixels and get a scan approximately 800 x 532,
leaving a bit of border on the top and bottom.

Now, what would I do?  The same as I mentioned in my earlier posting.
If the flatbed scanner has a native optical resolution of 600 dpi, I'd
use that, making a 3600 pixel by 2400 pixel scan from a 4 x6 print.

I'd then downsample it to 133, or 150 dpi bicubically, and unsharp mask
it, and then jpeg it to an optimum file size.

One final comment.  It is pretty much accepted in the industry that drug
store 4 x 6" snaps are produced to about 200 dpi resolution.  Therefore,
using much higher resolutions will likely not produce extra information
on the scan, but it is still of some value to use the full native
resolution of a scanner as the starting point of an image to be
downsampled.

Art



Ken Durling wrote:

  > On Mon, 22 Oct 2001 15:49:22 -0400, you wrote:
  >
  >
  >>>Hold on - thanks to you all, maybe I DO understand this.  If scanned
  >>>at 72 dpi, even a 4x6 print would need quite a bit of interpolation to
  >>>get it up to a good screen size, ergo crap.    Is that correct?
  >>>
  >>No, not interpolation.  Interpolation ADDS data.  Decimation removes
data,
  >>so scanning at 72dpi would remove data...if your scanner is 2700DPI
and you
  >>scan at 72DPI, you are only using 1 for every 37.5 pixels!
  >>
  >>Are you scanning prints?
  >>
  >
  >
  > On my flatbed, yes.  Usually at 150 dpi.  But  now with the FS2710
  > obviously I'm only doing slides and negs, which is what brought up all
  > these questions.
  >
  > I guess I'm missing the point here.  If I were to scan even a 4x6
  > print at 72 dpi, and then want to display it anything larger than
  > 288x432 pixels, wouldn't interpolation be necessary?  Even more with a
  > slide or a negative?
  >
  >
  > Ken
  >
  > .
  >
  >