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[filmscanners] Re: Topic: "Halo" Effect



Hi Joe,

I am not questioning your results, but I am questioning your methods
which lead you to the conclusion that the scanner is the issue.

I looked at your images, and see what you are discussing, but you state
that the same phenomenon is not visible with negative images made with
the same optical set up.  This, in itself, does not prove the scanner is
the issue.

What might prove it is if the halos you see aren't on the slide when you
look at it via some other method of viewing, such as a projected image
using a quality Navitar slide projector lens, or studying them with a
well color corrected microscope.

Slides and negative films respond differently to exactly this kind of
lighting situation.  The reason negs have a dye masking layer to to help
them to compensate for light scattering in high contrast, and to help
"fix" the inaccuracy in the emulsion filters which allow for light which
should be blocked to get through.  Some specialty negative films carry
an even more specialized "anti halation" layer, which is a black coating
on the back which has to be removed mechanically, which absorbs stray
photons which otherwise bounce back into the film.  But even standard
color neg films contain some anti halation qualities via the dye
masking.  Also, negative films lower contrast, and this also lowers the
degree of halation which would occur.

Making the assumption that because the halos are visible with you SS4000
with slides and not negs, does not prove the source is the scanner, in
fact, it might suggest the opposite (;-)).  You need to check those
slides with another type of optical system and make sure those halos
aren't indeed on the slide, caused by the telescope optics.  Negative
film may just be more forgiving of recording halos that slide film
records, so it may simply be the nature of the way the film is responding.

You may indeed be correct that the slide scanner is doing it (I don't
shoot these kind of images, so my experience with slides with this type
of contrast is very rare), but the testing needs to be done differently
to isolate that.

Art


Joe Pedit wrote:

>>Topic: "Halo" Effect
>>===============
>>Date: Sat, 17 Aug 2002 06:46:47 -0400
>>From: "Robert DeCandido, PhD" <rdcny@earthlink.net>
>>----------------------------------------
>>Hello All,
>>
>>I have a Polaroid Sprintscan 4000 (not the Plus version) and am using
>>Vuescan.  When I scan a slide (either Kodachrome or Provia/35mm), the
>>white areas (such as a building illuminated by the sun; or pages of an
>>open book) in the scan will exhibit a "halo" effect.  This appears as
>>a kind of a whitish or even greenish glow surrounding the white object
>>in the scan.
>>
>>My questions are: Is anyone else seeing this or getting this effect on
>>their scans?  Is this something gone wrong with the scanner?  Is it
>>something that different scan settings in Vuescan can correct?
>>
>>Using Knockout 2.0 I can correct most if not all of the "halo" or
>>"after glow."  However, if someone can set me straight regarding how
>>to solve the problem before the scan, I would be most appreciative.
>>
>>Thanks
>>
>>Robert DeCandido
>>NYC
>>
>>
>
>
> Hi Robert,
>
>
> I have observed similar "halo" effects from slides scanned with the
> Sprintscan 4000.  The size of the halos increase with vertical distance
> from the center of the slide.  The effect is best demonstrated by
> example.  The first image is the full image (reduced to 12.5% or 679x462
> pixels, 58kb):
> <http://www.unc.edu/~pedit/full.jpg>
> The next image is a star near the bottom of the image (expanded to 200%
> or 350x350 pixels, 43kb)
> <http://www.unc.edu/~pedit/bottom.jpg>
> The final image is a star near the top of the image (expanded to 200% or
> 350x350 pixels, 45kb)
> <http://www.unc.edu/~pedit/top.jpg>
> Notice how the red flares to the left and blue flares to the right.
> Stars near the vertical center do not have flares.  The effect is not
> observed on similar images from negatives acquired through the same
> telescope/camera combination.
>
> Joe
>
>
>


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