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Re: [APML] Blue response on astrophotos
Jason,
Very nice shots of M42. I admire anyone who can guide successfully at
longer focal lengths like 2000mm! I am happy if I can do a good job at
600mm f.l. Your colour balance is good, especially the "first" version.
I wouldn't argue that Provia 400F doesn't have well balanced, even
colours overall; just that it does not seem _especially_ strong in the
blue, which is what I'd been led to believe by other sources.
Most of Provia's speed and sensitivity seems to be in the green and red
channels, but more the orange side of red than the deep H-alpha side. So
it is particularly good at picking up airglow & light pollution lines,
which is unfortunate! It seems to me in my tests that E200, although 1
stop slower, has (in absolute terms) the same or better net H-alpha
sensitivity, less green/orange airglow sensitivity, and about the same
blue. I need to do more tests, but so far I prefer E200 for Milky Way
and nebula shots. Maybe it's because I am the kind of astrophotographer
who sometimes doesn't want even colour sensitivity - I want very biased
sensitivity, up for nebulae and down for airglow! If you want to
digitally boost the red of H-alpha nebulae in Provia shots, you can end
up making the stars too red also. This is not a problem with E200, which
naturally picks up a great excess of narrowband H-alpha over broadband Red.
I am also trying to get my hands on some Centuria 400 - in 120 format. I
have a possible lead in Poland (it could be a misprint however!), so my
Polish friend's girlfriend is over there and going to check it out for me.
Ray "who hasn't shot deep sky for nearly 4 months - with the triple
bogey of weather, work and perma-twilight" Butler
Lane, Jason R wrote:
> 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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