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Re: filmscanners: Figuring out size & resolution

Rick, I'm not sure exactly what you mean by "density," but not knowing what
the question is has never stopped me from giving an answer before.  I'll
assume that density means the number of dots per inch (dpi) that you send to
the printer.  There's another value that has to do with how many dpi the
printer actually prints on paper, such as 1440 dpi.  But that value is
printer specific.  My Epson 2000P doesn't even let me set that value.  It
gives me a choice of printing at "photo quality" speed or "high speed" and
adjusts the number of "dots" on the paper accordingly.  My recommendation is
that you tell your printer to print on paper at the highest number of dots
per inch your printer is capable of (1440?) so as to get the best "photo
quality" and that you send the image to the printer at 300 dpi.  I'd also
recommend you check out www.scantips.com for related information.

Most printers are happy if they are fed data at a "density" of 300 dpi.  With
less than that the print quality suffers.  With more dpi than that, it's just
a waste of good pixels and the print quality isn't any better than if 300 dpi
were used.  I've read on this list that some of the cheaper printers don't
improve past about 240 dpi and there are some that don't stop improving until
you pass 360 dpi.  But a good rule of thumb is to use something close to 300

For example, assume that you scan a 35 mm slide with a scanner that scans at
4000 dpi.  If the slide is 1x1.5 inches in size (it's a bit smaller due to
the mount, etc., but this makes the math easier), then you'll have an image
that's 4000x6000 pixels.  The maximum size of print you can make from that
image is found by dividing the 4000 and 6000 pixel dimensions by 300 (the dpi
you need to send to your printer).  And the answer is:  13.33x20.00 inches.  
So that's the largest print that you can get and still have at least 300 dpi
worth of data to send to your printer.  If you try to print something bigger
than that, you'll be forced to send less than 300 dpi to the printer and the
print quality will go down.  But, suppose you want to an 8x12 print.  Divide
4000 by 8 (or 6000 by 12) and you find that you'll be sending 500 dpi to the
printer.  That's more dpi than you really need, but it won't hurt anything.  
(I've heard on this list that using more dpi than necessary uses more ink,
but I don't think that's true.  Maybe someone on the list can enlighten me.)  
You can do your calculations in reverse, also.  For example, if you want to
make an 11x14, then multiply each of those numbers by 300 and you'll find
that you need an image that's at least 3300x4200 pixels in size.  So if you
have a 4000x6000 pixel image, then you've got more than enough pixels for a
good print.  In fact, you might want to "throw away" some of the unneeded
pixels if you want to save a copy of the print on your hard drive since the
file size would be smaller.  To do that you resize the 4000 dpi, 4000x6000
pixels, 1x1.5 inch original image so that it becomes 300 dpi, 3300x4200
pixels, and 8x12 inches.  Remember, though, that once you resize an image in
such a manner that pixels are thrown away, you can't resize back up to an
image with the original number of pixels.

I hope I answered your question.  If not, restate it and I, or someone else,
will try again.

In a message dated 7/7/2001 12:06:08 PM Pacific Daylight Time,
rickdecker@hawaii.rr.com writes:

I just bought a Epson 1270SU.

Is there a formula for picking density and output size based on input
size and projected print size.

I scan 6x7 and will print either 8x10 or 11x14

And I scan 35mm and will print either 8x(10/12) or 11x(14/16)

If I specify my output size, how do I decide what density to pick?


Rick Decker


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