Re: [APML] Alternative Anti-Vignetting Photoshop Technique

[Matt BenDaniel <matt@starmatt.com>]

George,

Thanks.

Generally I do AV by manually painting only shades of gray in a screen
layer.  I put a linked curves layer on top of the screen layer, which
compensates for the color shift toward the corners.  Only as a last
resort do I actually paint non-gray shades into the screen layer.  

Most people have a justifiable aversion to manually painting layers.  It
takes time, and if done incorrectly, it will introduce subjective
artifacts.  But my assessment is:
* It works better than any other AV method I have seen.
* Unfortunately, I have not been able to reuse AV layers from one shot
to another, without manual customization.

One of the most interesting realizations I had last year was that 400F
film does NOT seem to have any color balance shift due to differential
reciprocity failure (and doesn't need a linked curves layer).  That
makes AV much easier.  It is the only film I've ever seen that has that
characteristic.  It is probably the best slide film for AP available
(and YAY it's available in medium format).  The current RASC magazine
has an article by Dickinson on astro films, and he says that 400F is the
best astro slide film.  I should have followed John Boudreau's lead
sooner when he told us about 400F last February.

My next experiments will be around whether 400F permits easy re-use of
AV layers from shot to shot. If it does, that would be a huge time
saver.

Matt

George Whitney wrote:
> 
> Matt,
> Thank you.  Those are some great points and suggestions.  I look forward to
> trying out your techniques on some shots.  I wonder if using the dodge/burn
> tool of Photoshop will correct for some of the color shift due to differing
> reciprocity of the film layers (dodge/burn the inverse color in the shadows,
> midtones, highlights)?
> Let me say that I thoroughly enjoy your images...their beauty provides me
> with inspiration.  Your "labor of love" has certainly paid off.  <G>
> George
> 
> -----Original Message-----
> From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
> Behalf Of Matt BenDaniel
> Sent: Monday, January 07, 2002 5:47 PM
> To: astro-photo@seds.org
> Subject: Re: [APML] Alternative Anti-Vignetting Photoshop Technique
> 
> George,
> 
> I'm sure the technique you describe provides some AV help to an image,
> but it is not a total solution.
> 
> * During a long exposure, reciprocity failure affects the film response,
> which is not replicated by a short flat-field exposure.
> * Differential reciprocity failure also significantly affects color
> balance.
> * Only a form of multiplication or screening provides a true inverse
> function of light falloff.
> 
> I use an AV technique which allows use of the entire frame, as long as
> the background is recorded above the toe in the corners.  For more info
> see:
> http://www.millennics.com/olympus/tope/tope1/pictures/tope1_17.jpg
> 
> My imaging system has fairly severe vignetting, but you will see few
> signs of it in my images.
> 
> The big problem with the technique I use is that it is labor intensive.
> But AP is a labor of love, right?
> 
> Matt
> 
> George Whitney wrote:
> > Jeff,
> > The best way to fix uneven illumination is by flat fielding, just like in
> > CCD photography.  Photograph an evenly light square of white foam core
> > through an opal glass using a continuous tone film, reverse the image in
> > Photoshop, and stack the result with your scanned image and...presto...no
> > vignetting!
> > George
> --
> Matt BenDaniel
> http://starmatt.com
> 
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-- 
Matt BenDaniel
http://starmatt.com

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