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[APML] SFL and Flats Was: Fuji Provia 400F and exposure time
> I have some different thought. First of all, I
> don't quite know what
> is the definition
> of "sky fog limit." I heard this very often but
> hasn't figured out
> what exactly it is.
As I undestand it, is the maximun time you can expose
where the sky's emision don't affects the faintest
nebulosities. If you go longer, the sky fog starts to
"eat" those information.
> Sometimes I don't plan to view the slide directly
> but I want to scan
> it and enhance it
> digitally. In this case, unless some part of the
> image may be
> saturated, I usually
> expose as long as I can to get the best S/N, which
> is essential for
> digital processing.
Well, from my experience, pushing the film makes
things easier when scanning, due the less dense
background.
> According to my test on Provia 400F and E200, the
> best S/N is reached at Zone VI
> exposures. What's zone VI? Shoot a grey card with
> +1 EV exposure. The deisty
> is Zone VI. A zone VI sky on a slide looks quite
> bright but it does
> contain the faintest
> information that a slide can record. Again, it
> requires some
> experience to tell which
> exposure time will give Zone VI.
>
Is the information recorded there in the linear zone
of film?
BTW, I have a question for you... about film
flatfielding: When you divide, Do you have any
problems with the star's intensities in the center? I
ask becouse when I divide with a background model
generated throught DBE the stars in the center of the
field become fainter than the ones in the corner.
Should I use a luminance mask for that? Or it would be
better to use some kind os luminance dependient
funcion when dividing?
BTW-BTW: How do you divide using your flats? Do you
make any previous threatment to them? Or you use them
"raw"?
=====
Regards,
Carlos Milovic F.
-------------------------
Astro & Photo - CMF
http://www.astrophoto.vze.com
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Visita "AstroFoto", el foro de astrofotografía en español
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