RE: [APML] Blue response on astrophotos

["Lane, Jason R" <jason.lane@navy.mil>]

I've always seen what I thought was a good color balance in Provia 400F.  You can reference this shot that I took in January:  http://www1.iwvisp.com/opticman/astroimages/M42_031004.html  The blue nebula at bottom is a little dimmer than what I had hoped for, but I believe that's due to vignetting in the setup.  Still, it looks pretty well balanced I'd think.   Maybe 400F is different when sold in Europe, or maybe they've changed the formula?  I'm working on a roll of Centuria 400 right now, but after it's done I can load some 400F and go out and shoot M20 area and see if I get similar results.

-Jason


-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Ray Butler
Sent: Tuesday, August 10, 2004 8:06 AM
To: Discussion of Film Astrophotography
Subject: Re: [APML] Blue response on astrophotos




Dear Nicola,

After shooting 10 rolls of Provia 400F (120-format), I find the same 
thing...I also have not been able to reproduce the reported strong blue 
sensitivity. I get at least as much blue in reflection nebulae from 
Kodak E200, and that emulsion is supposed to be quite poor in the blue. 
So with E200's considerably better H-alpha peak, I can't find any reason 
to keep using the Provia - except that it is faster overall for 
starfields, comets and aurorae.

The only thing I can think of: is the Provia sold here in Europe 
different to that sold elsewhere?

As to the solutions: if you scan the slides, you can manipulate the 
colour balance in software. There are too many possibilities here to 
give specific details, but you basically want to increase the blue 
emphasis _without_ changing the blue point of the sky background. That 
is the main thing to avoid. Whether you do some manual, nonlinear 
"curves" adjustment (which personally I disapprove of...!) or a linear 
transformation of the dynamic range above the background level (a lot 
more objective and scientific) is up to you.

Alternatively, you could put a "colour correction" (light blue eg. 82A) 
filter on the lens/telescope when taking the shot. Regular photographers 
use these to correct the reddish colour-temperature of some light 
sources, like the setting sun. The downside is longer exposure time 
needed to compensate for the absorbed red and green light - the filter 
factor is 1.1 (+1/3 stop) and up, depending on how deep blue the filter is.

Your website is very nice, by the way. That Yashica 200mm f4 lens is a 
fine performer. Reminds me of my Zeiss Jena 300mm f4 on 645 and 6x6.

Ray "just back from a run on the 1.5m at Loiano Observatory, Italy" Butler


centaurus_a@virgilio.it wrote:
> Dear All,
> 
> I recently shot M20 with Fuji Provia 400F and I noticed something that I
> can't explain. The red part of the nebula is standing nicely while the blue
> part is barely visible. Can anyone explain me why? It is said that Fuji
> provia as good colour balance, so why I get less response to blue than to
> red?
> Do you know a selective way to enhance the blue keeping the rest of the
> shot correctly balanced?
> 
> Regards,
> Nicola
> http://astrofoto.laza.it
> 
> _______________________________________________
> Astro-Photo mailing list
> Astro-Photo@seds.org
> http://seds.org/mailman/listinfo/astro-photo

-- 
Dr. Ray Butler (ray.butler@nuigalway.ie || ray@physics.nuigalway.ie)
Lecturer, Dept. of Physics || Computational Astrophysics Laboratory
National University of Ireland, Galway, University Road, Galway, Ireland.
Tel: +353-91-524411 ext. 3788   FAX: +353-91-525700

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