<< With this 720x720 printer, what is the minimum resolution of
a scan that is needed to get the best results? At some point (200dpi?) a
higher resolution scan will not increase the quality of the output. >>
This is not an easy question to answer. With screening, you can easily
calculate the trade-off between lpi and level of grays (or colors), but most
color printers do not use screening technology for printing because the
resolution is too low for that (yes, 720 is extremely low resolution). What
they do is to use dithering. There are different algorithms for dithering (and
newer ones keep coming out), so you don't know which one your printer is
using.
The basic idea is similar to diffusion transfer where you approximate a pixel
level and then carry over the error to the next pixel. The advantage is that
you can get much smoother gradation (instead of seeing big halftone dots if
you were to use a 720 dpi printer for screening). The disadvantages is that
you lose details (because of cross talk between pixels). You can of course
make your image look sharp by composition, sharpening, etc., but basically
only apparent sharpness is increased, not the actual details.
Dave Soemarko