On Mon, 18 Jun 2001 07:02:45 -0700 Shough, Dean (dean.shough@lmco.com)
wrote:
> That is what MTFs (Modulation Transfer Function) are for. The MTF for
> optical systems can be either computed (see Canon's EF Lens Work) or
> measured.
Yes I know this, it is what I was referring to without calling it MTF -
and my point was that Nyquist renders MTF incalculable for pixel-based
*systems*. It is also difficult to measure sensibly, as the position and
orientation of a conventional MTF target relative to the pixel locations
affects the amount of aliasing and consequent artefacts.
About all you can determine easily is the theoretical MTF
possible at the CCD, according to the Nyquist limit, which tells you about
as much about scanner system performance as an MTF test of film does about
a camera/film system. We can't rip the lens out of these things and test
them separately either.
There are special resolution targets available for empirical determination
of pixel-based MTF - eg for targets made for
scientific use - but I am not aware of any which are suitable for 35mm or
other filmscanners. Sinepatterns would doubtless make one for a few
thousand $$, but that is somewhat beyond my means;)
Regards
Tony Sleep
- Online portfolio & exhibit; + film scanner
info & comparisons
