Kennedy's point about honoring the capabilities of your scanner,
however, is well taken. In Preview at the bottom of the screen, it
shows the pixel dimensions of your image. Your scanner scans vertically
at 2400 dpi. You check the pixel width at the bottom of the screen to
make sure it does not exceed 2400 dpi. If it does, you need to change
your Target dimensions.
If you are looking for the best results from your scanner and printer
then you simply cannot just leave it to the scanner software - even if
you do check that your combination of image size and output ppi does not
exceed the scanner's capability.
The scanner produces one image pixel for every CCD element at its
optical resolution only. At almost every other scan resolution, the
result is interpolated - and usually interpolated by the crudest of
algorithms, nearest neighbour in most cases and bilinear interpolation
in a few. In fact, it is actually worse than that because, in the
interest of scan speed, the interpolation is usually applied to the next
highest integer division of the scanner resolution, so an amount of
information equal to the square of that integer is not even considered
in the interpolation!
If you scan at the optical resolution of the scanner and then scale the
image at printing time, you can choose the interpolation that you use to
get the size and resolution you want for your final image. There is a
vast array of interpolation algorithms that are vastly superior to
nearest neighbour or linear interpolation, and several of these are
built into standard image editing and printing software. They are not
generally offered in scanner software because of the impact on scan time
- the scanner manufacturer, on the advice of millions of users, wants to
offer the maximum scan speed possible.
With original source material as small as a 35mm frame and the resulting
high magnifications in the final output, such issues are important.
It is also completely wrong to suggest that the printing technology
available today is unlikely to change sufficiently to impact the
intended use. Quite simply, printing technology doesn't have to change
at all - all you need to do is buy another manufacturer's printer and
the optimum output resolution is likely to change!
For example, current HP printers are optimised for output resolutions
which are an integer division of 600ppi, the native raster resolution of
the printer driver, hence the typical advice to target an output
resolution of 300ppi. 200, 150 and 120ppi are the next lowest
resolutions of choice for HP technology. However, 300ppi is actually
one of the worst resolutions to select for an Epson printer.
Epson desktop printer drivers have a native resolution of 720ppi, so the
optimum resolution for the output is an integer division of 720ppi.
360ppi is the Epson equivalent of the general "300ppi" rule, whilst 240,
180 and 144ppi are just the next lowest resolutions of choice. Most
Epson large format, generally professional, printers have a native
driver resolution of 360ppi although some of the latest use 720ppi
drivers just like the desktops.
Anything other than these integer divisions of the native resolutions
requires interpolation in the printer driver (in addition to what you
might have used in the scanner driver!) and, guess what, they are even
more limited than the scanner drivers in that they *all* use nearest
neighbour interpolation! (Some Epson drives do have an option to enable
linear interpolation, but unless you know what is going on, it is so
poorly labelled that few know what it is and it is frequently misused).
300ppi images fed to an Epson printer guarantees that the output is
interpolated, with the results most obvious on sharp edges which are
near vertical or horizontal. Admittedly, text and synthetic graphics
show the effect more than scanned images, but it is still there, can
still be seen and all of these interpolation steps add up to a poorer
result.
To avoid any confusion, note that the optimum printer ppi figures are
virtually independent of the quoted ink dpi for the printer, which is
generally an integer *multiple* of the driver's native raster resolution
and determined by the design and manufacturing technology used.
Your optimum image requirements can change simply by changing between
printers that are freely available on the market today, let alone
guessing about what might be available in the future!
Now, so far on this thread, the OP has been advised to target a specific
output resolution - which forces interpolation at scan time and may
force yet another interpolation at print time, depending on the printer
being used. And if he wants to perform any crops on those scanned
images before printing them at the same size, yet another interpolation
is required. Just how many stages of the poorest type of interpolation
do you think are needed before the output looks like mush? At most, one
interpolation step is all that is necessary, and the effect of that can
be minimised by using the best interpolation algorithm available in your
armoury. So why are you recommending a workflow that could result in
three, two of which are the worst possible type?
Based on the statement that the OP has insufficient scanner resolution
to achieve even 300ppi at the output size he wants, do you know whether
he should be targeting 200ppi or 240ppi as his next best choice. And
can you guarantee that this will still be his optimum output resolution
next week, month or year?
It is a poor shortcut to impose printing criteria on a scanner or
scanner criteria on a printer. Separate the two functions, choose the
optimum for each and use the best interpolation technology available to
you at any time to bridge the gap between them.
Or, as someone allegedly more savvy than anyone on Usenet put it:
"Render unto Caesar that which is Caesar's, and unto God that which is
God's".
