Re: [APML] Supra 400 Reciprocity

[astropix@att.net (Geoff Powers)]

 On Wed, 12 May 2004 06:08:21 +0000, Don Westergren writes:
> Supra 400 can do it. It has some reciprocity in the first hour,
> then almost no further losses.  See my data on Supra at
> http://home.nethere.net/mpd/FilmTestData/KodakSupra400.htm

 Hey Steve...

     Don's data is not only empirical, but also tallies with 'real world' experience... at least my own. Print films with good reciprocity have exceptional dynamic range capabilities... but usually take longer than slide films to reach their 'toe' of exposure sensitivities. 

     Any film starts slow to expose until it hits the 'toe'... after a film hits the 'toe' of it's exposure capabilities, the light recording effect is almost linear until the 'sky fog' limit is achieved. 

     The fly in this ointment is that reciprocity failure keeps many films from ever hitting the 'toe' in an astronomical exposure... no 'toe' = no sky fog... or a totally skewed color response from differential reciprocity failure among the separate color sensitive layers in the film's emulsion.

     Slide films with long exposure capabilities (e.g. low reciprocity failure) hit their 'toe' much faster, but also have a smaller exposure latitude. Expose slides just a little too long, all contrast is gone. Even with a 'good' astro exposure, slides will still 'burn out' or generate 'halation' of the brightest objects (e.g. stars) in the scene.

     Print film is much more forgiving of bright objects in an astronomical scene, but takes considerably longer to hit it's 'toe'. After print film hits the 'toe', it records additional faint spatial data, without over-saturating the bright stuff that's already there... if said film has good reciprocity characteristics to begin with. 

     The original 'Supra' emulsion will go to it's 'toe'... and keep recording until daylight. The Konica 'Centurion' 400 and 800 speeds seem to show similar performance. I have been working with advanced post-processing for print films, using Picture Window Pro 3.5.x... the preliminary efforts may be viewed here:

      http://home.att.net/~astropix/guide_PW_Pro_3.5.html

... you may want to look at the first section closely, 'Scanning For Dynamic Range'... and then look at how 'Color/Negative' converts the scan. The 'Histograms' with this technique are just *awesome*... easy to stretch after their conversion to a positive, color image. Apologies for not having this guide finished... still experimenting !!!

 Best...   :o)

    
--
 Geoff

http://home.att.net/~astropix/


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