alex8735 schrieb:

For post processing it is important to have a high resolution.

Agreed. However, it's all a question of economics. There's definitely no
point in scanning an entire magazine page with 1200 dpi, which will
create something like 500 MB of data, then sending this data through a
modem line, then working on this huge file, only to have it downscaled
at the end to something which will be displayed as a 15cm x 10cm picture
on a web site. Scan as needed.

I am quite sure that National Geographic prints at a higher resolution
than 300dpi (my printer at home does 600dpi).

For line art, high resolution is crucial. I can even tell the difference
between 600dpi and 1200dpi.

However for half tones (aka pictures), there's no use to set the
resolution higher than twice the printed screen frequency. Note: This is
the *theoretic* maximum of data that can be reproduced when you
rasterize a picture! Practically, with most pictures, even a factor 1.4
will yield perfect results.

Now as most magazines print with a screen of around 150lpi, the raw
material is never higher than 300dpi. (Art reproduction is a different
story because they use finer screens. And FM rasters are yet a different
story altogether.)

I still recommend not to use any kind of scanner demoiré features
because scanner software usually is not as good as Photoshop.

Replace "usually" by "sometimes", the I agree. Again, it's a question of
economics: If you want to demoiré in Photoshop, you indeed need a high
resolution scan.

Stefan