[filmscanners] Re: PS sharpening

["Preben S. Kristensen" <krille@tin.it>]

I find this an interesting and utterly relevant subject, but I do not have
the time to research it further as I am travelling.
Any takers?

Kind regards   Preben

From: "Anthony Atkielski" <anthony@atkielski.com>

snip<

I normally USM on the initial image, then after each downsampling, except
possibly for the last (depending on how large a step the last downsampling
is, the result may or may not look better with a final USM).

One of these days I'll have to try to do some controlled experiments to see
if the multiple-step process is provably superior to a single-step process.
I think there is probably a single-step equivalent, in any case, but it
might require more careful setting of USM parameters and they might have to
be set individually for each image. >snip
--------------------------------------------------------------

Julian Vrieslander wrote:

Snip<
Earlier in this thread (or elsewhere) someone offered an explanation for the
alleged advantage of multistep sharpening: if you apply a slight
oversharpening before each downsampling operation, you preserve details that
might be lost when pixels are tossed.  But then I read a note from Kennedy
McEwen on another list, where he explained that for a theoretically
optimized downsampling you should apply a blur before the downsampling, to
reduce the effects of aliasing.

How do we reconcile this?


n. b.:  The blur prescribed by Kennedy was small.  If gaussian blur is used,
the params are:

radius = (0.3 x S)/(S - E), where S is start res, and E is end res.

For example, r = 0.6 pixels before downsampling from 4000 to 2000ppi
>Snip





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