SNIP

That would depend on:
1. The amount of sharpening before scaling down, if any, and
2. the resampling method and amount used for scaling down, and
3. the sharpening needed to compensate for resampling losses.

A scan of a theoretically perfect image can be mimicked by scanning a 5
degree slanted sharp edge, somewhat similar to the principle used by the ISO
to determine scanner resolution. A home made solution is by mounting a razor
blade in a 35mm slide mount. Another way is by mounting a piece of
aluminum/aluminium foil, folded once to form a straight edge and flattened
with careful pressure. The slant will reduce the differences between more or
less perfect alignment with the sensor array.

That will produce a target that is not yet blurred by a camera lens, so only
the scanner can introduce deterioration. After scanning you'll also notice
the amount of lens flare and internal reflection caused by the scanner
(surround the slide mount with a full glass platen mask for best results on
a flatbed scanner). Try and avoid blooming due to overexposure of the
unobstructed view of the lightsource. Then you can draw several across edge
luminance profiles (try two 90 degrees rotated scans to have both a sensor
and a stepper motor resolution) and count the number of pixels it takes e.g.
between 90% of maximum Luminance and 10% above minimum Luminance close to
the edge on the Raw gamma 1.0 data.

Try several sharpening radii with a modest amount to see which helps to
improve the steepnes of the transition, and use that for the above step 1.
Then use a resampling method that makes sense for the intended image
content, using an amount to reduce the number of transition pixels to
between two and three. Finally, sharpen to compensate for resampling losses.
That should give the best quality, and largest possible uncompromised scan
size your scanner can produce.

Applying the same steps to a film scan should also produce good quality, but
one may want to deviate a bit based on film characteristics and intended
use. If e.g. one finally needs to enlarge again for output, then why reduce
size first, and if the final image needs to be reduced further then it is
better not to resample twice. Final sharpening should be based on final
output size.

Testing the performance as described above will give a good feeling what is
needed to approach the best possible 10%-90% Luminance difference transition
width in a real image. Lower image contrasts will only lose visible contrast
faster, but the width of the transition is never larger.

As an example, my Epson 2450 @ 2400 ppi takes 10 pixels to cross the edge,
and after USM Amount 30, Radius 3.0, Threshold 0, that is reduced to 5.
Reducing that Image size to 50% gives me a 2-3 pixel transition which can be
further small radius sharpened to exaggerate the edge transition for a
visually pleasing result. A bit more sharpening in the beginning is also
possible.
In fact these settings increased the resolution from 31.5 cy/mm to 50.1
cy/mm on a sample of Fuji RA film I had scanned long ago, allowing a pretty
decent 5-6x enlargement of the film.

Bart