BKPhoto@aol.com
Date: 11/09/01-04:28:04 PM Z
Group,
Photoshop translates tonal values/density values/zone values--whatever you
want to call them--into 256 code values (0-255, you can see this in Levels or
Curves). Imagine each CV as a bucket. If your histogram is solid, all 256
buckets contain information. If you see gaps (white lines), there are empty
buckets. Interpolation, by the way, is one way of trying to fill those
buckets; not with image data, but with data invented by program code based on
image data.
A 16-bit file has the advantage of putting more digital information into any
given bucket. It does not, cannot, create more buckets. Everything else being
equal, if you want to produce a digital print or negative a 16-bit file is
preferable to an 8-bit file (regardless of how information is processed by
the printer driver). However, a 16-bit file will not necessarily prevent the
contouring you sometimes see, especially in areas of continuos tonality. This
issue is directly related to the optical resolution of the scanner and, more
importantly, the optical density range of the scanner. And, as already
pointed out, it can be massively effected by how much the digital information
in an image file is processed. Every Photoshop action or procedure that
destroys information--and most of them do--can contribute to contouring,
among other problems. That is why it is best to develop a Photoshop
methodology that gives you the best printed image possible (positive or
negative) in the fewest steps possible.
A 16-bit file helps, but it isn't the complete answer. Contouring is a
serious problem. In fact, it serves as a very good indicator of the "health"
of any given digital imaging system.
Bill Kennedy
Austin, Texas
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