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Re: filmscanners: Film Scanner Question Again

Rick, I'm not familiar with your scanner, but I'm going to pretend that I
know what I'm talking about.  So fasten your seat belt; this may be a bumpy

Another post indicated, if I read it correctly, that your scanner has a
maximum optical resolution of 3200 ppi in one direction and 1600 ppi in
another.  As others have stated, it's almost always best to scan at the
maximum optical resolution of the scanner.  You can always throw away extra
pixels later if you don't need them.  So, in your case, it looks you should
scan at 3200 ppi.  The scanner is going to "pad" one scan axis (the one that
can only scan at 1600 ppi optically) with some extra pixels by making an
educated guess (interpolating) at what they should be, but at least you'll
get all of the data out of the 3200 ppi axis.  My guess is that, of the 3
scanner parameters you need to set, "DPI" is the one that should be set to

So that leaves the "Source Size" and "Target Size" parameters.  I'd set both
to the size of the film being scanned, such as 1x1.5 inches for a 35 mm
slide.  With most scanners I'm familiar with, when you get ready to scan
something, you first do a "prescan" and then adjust the sides of a crop box
so that it includes only that part that you want to scan.  After you've set
the crop lines, then you do the actual scan and only that part that you
selected with the crop lines gets scanned. In the process of setting the crop
lines, I suspect that the "Source Size" will be automatically set for you.  
You have a fourth parameter you alluded to, that being "Scale."  Set it to
100 per cent.  I suspect that when you do that, it will cause the "Target
Size" to automatically be set the value of the "Source Size."  As I said
before, I don't know what I'm talking about because I'm not familiar with
your scanner, so none of what I'm telling you may be accurate.  But a lot of
scanners work the way I've explained and there is a lot of interaction
between the three or four parameters.  If your scanner doesn't allow for a
prescan, then you'll have to set "Source Size" yourself.  Set it just big
enough so that all of your film gets scanned.  In this case, you might very
well have a larger than normal file because you had to scan in a lot of
useless area around the film.  It's not a problem.  Simply use Photoshop (or
whatever equivalent software you are using) to crop off the unnecessary
stuff, and the file size will drop to a manageable size.

So, in summary, scan at the highest optical resolution of your scanner (3200
ppi), set Source Size and Target Size equal to the film size being scanned,
and set Scale to 100 per cent.  Then, if you look at the image in Photoshop,
you'll see that it is about 1x1.5 inches (for a 35 mm film scan) and has a
resolution of 3200 ppi and has a pixel size of about 3200x4800 pixels.  Do
whatever you want to in Photoshop, then save a copy to your hard drive.  
Then, use Photoshop to resize the image before printing to, say 8x12 inches,
and, at the same time, change the Photoshop resolution to one-eighth (because
you increasing the size by a factor of eight) of its original 3200 ppi
resolution, or 400 ppi.  So now Photoshop has an image that's 8x12 inches,
400 ppi resolution, and still 3200x4800 pixels.  (You haven't created, or
thrown away, any pixels by resizing, which is usually a Good Thing.)  As I
mentioned in a previous post, you want to sent the printer at least 300 ppi
data, so 400 ppi is more than enough for an 8x12.

Boy, I hope I'm guessing correctly as to how your scanner works and that this
is going to be of some help to you.  Remember, you can always try different
combinations of settings and look at the output from your printer to see what
works and what doesn't.

One final comment.  Your scanner has a maximum resolution of 3200 ppi in only
one axis.  It's half that in the other.  So, if you scan at 3200 ppi, one
axis of the image is going to be half as sharp as its 3200 ppi resolution
implies.  We've said that you want at least 300 ppi to send to the printer,
but since your image isn't as sharp along one axis as we've been assuming,
you might want to send more than 300 ppi to the printer.  How much more?  
Well, it won't be any more than 600 ppi since that's the value we'd use if
both axes had optical resolutions of 1600 and we scanned at 3200.  So the
correct value is between 300 and 600 ppi.  I did some fancy math involving
root-mean-square and determine that you should send 474 ppi data to the
printer when (and only when) you scan at 3200 ppi where one axis has a
resolution of half that.  This means that, for a 8x12 inch print from a 1x1.5
inch piece of film, the 400 ppi that we have available falls short of the 474
ppi that we would like to have.  Don't worry about it.  I doubt it'll make
all that much difference.  But it does indicate that an 8x12 from 35 mm film
is a marginal situation for your scanner and you certainly wouldn't want to
print anything much larger than that.

Let us know if any of this helps.  

In a message dated 7/7/2001 6:43:09 PM Pacific Daylight Time,
rickdecker@hawaii.rr.com writes:

I have 3 parameters on my 1640SU scanner - "Source Size" , "Target Size"
and "DPI".  The manual tells me to "Increase Resolution" as I increase
Target Size.  This can result in some horrendously large files and I
suspect there is a limit above which the file gets bigger but does not
contain any more data.  The manual talks about 300 dpi resolution and
higher as the target sizes increase.  The question I have is if my
target is 8x12 for a 35mm slide what should my resolution be (DPI).
Here is what the reference manual says "You must manually adjust the
resolution (DPI) to maintain the same image quality when you resize your
image.  For example, if you have a 2x2 300 dpi image that you want to
enlarge to 4x4 (a factor of 200%), increase the Scale setting to 200%
and change the resolution to 600 dpi".

Now let's say that I want to take a 35mm slide (1x1.5in) and enlarge it
to 8x12...my resolution would be 2400 according to the manua
(12/1.5=8...8/1=8...300x8=2400)l.  At 1600, I would have a file size of
705 megabytes!! I am sure that this is way beyond the point at which the
increase in file size does not result in any more increase in data.

It sounds to me like I should leave my resolution at 300 dpi/ppi...even
that will give me a file size of 24megabytes which I suspect is larger
than I need.  For an 11x16 it would be 45 megabytes.

Any advice is much appreciated.


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