RE: filmscanners: Sharpness of color chrome vs color negative.

[Mike Duncan <mgduncan@esper.com>]

>Fuji does provide MTF for both their positive and negative film, Provia 100
>and Superia 100.  There is also additional information besides the MTF
>provided, like resolving power and diffuse RMS granularity value.  The
>problem I am having with the numbers provided, is I must not be
>understanding them completely.
>
>I know that the RMS granularity for chrome film has to be divided by
>2.5...that would make them equal (Provia is 10, divided by 2.5 = 4, and
>Superia is 4.  OK, so that's a wash...  Then, the resolution shows that at
>1000:1, Provia resolves 140 lines/mm and Superia 125.  That would mean that
>Provia (positive film) has a HIGHER resolving power than Superia (negative
>film)...BUT, wait...there's more.
>
>The MTF for Provia falls off at around 60, and the MTF for Superia falls off
>at ~130.  That, to me, means Superia is FAR superior at resolution than
>Provia....  OK, so why the discrepancy?

Sharpness is determined by the 50% point and resolution is determined by
just visible lines (typically 5% to 10%).  If the 50% frequency (fc) is
60cycles/mm, the the 10% to 90% transition width of a white-black step will
be

2.2/(2*pi*fc)=0.0058 mm.

Mike Duncan