RE: Digital Negatives

Elton N. Kaufmann - Cycloid Fathom Group (cycloid@cycloid-fathom.com)
Tue, 24 Feb 1998 09:34:32 -0600

This post seems to be explaining how Moire patterns can arise when digital
patterns interact. Is not "banding" simply due to too few intensity levels
in the filtering process (analog to digital conversion) to truly duplicate
continuous tone? Or is the term "banding" used for both effects?
EltonK

At 06:28 AM 2/24/98 -0800, Beakman wrote:
>> I, too have been working with Island Imaging. Scott Blaylock is
>> definitely a great guy to work with. He REALLY wants to make this work.
>> However, I am using Fokos' method for digital negs, rather than
>> Burkholder's. So far, they have been experiencing banding problems with
>> this output and are still trying to work out a fix. I'll post again to
>> the list as soon as I get a good negative from them. I think it's only
>> a matter of time.
>>
>> Kerik
>
>
>I am assuming that the banding you are experiencing is primarily in the
>highlights. The key here is to:
>
>a) tweak the compensation curve to match your particular paper/coating
>method/developer combination
>
>and
>
>b) get the exposure of the digital negative correct. You may think "but
>they're just black dots", but this is not true. The negative acts like a
>hybrid analog/digital negative and it is important that the density of
>the negative be correct in order to mate correctly with the
>characteristic curve of your paper.
>
>Because you are trying to match a an irregular curve moving a little in one
>direction or antother along the curve will create a mismatch, and hence
>banding.
>
>To help you visualize this, consider a graph of the characteristic curve
>of your paper. Next, cut a piece of cardboard to match this curve. Mark the
>10% point on the cardboard curve. Now move align the point you just
>marked with the 15% point on the graph -- this represents making a
>digital negative too dense (for a correctly exposed negative the 10% mark
>of both the cardboard and the graphe would line up). With the 10% and
>15% marks aligned you will note a curve mismatch -- you won't be able to
>get both curves to match along their entire lengths. The points where
>the two curves diverge can lead to banding.
>
>c) Banding can sometimes be caused by the type of line screen used --
>round dot, for exapmple has banding near 50%, elliptical dot in other
>areas. The differently shaped dots were all developed to help this
>problem. There are even some special linescreen patterns which change
>shape depending on the density.
>
>d) Print so that your 1% dot prints paper white. Do not have any 0% dot
>areas in your negative.
>
>e) In my image file I make the very lightest, non-pure-white tone have at
>least a 7% density (this is before the platinum compensation curve is
>applied).
>
>Good luck!
>
>David
>
>
Exhibiting fine-art and editorial photography
Cycloid Fathom Gallery
Elton N. Kaufmann, Chairman
P. O. Box 8129
Downers Grove, Illinois 60517-8129
USA
cycloid@cycloid-fathom.com
http://www.cycloid-fathom.com/gallery/
ftp.cycloid-fathom.com/group/uploads/