Re: [APML] Supra 400 Reciprocity

[astropix@att.net (Geoff Powers)]

 On Fri, 14 May 2004 02:45:15 +0000, Phillip Jones writes:
> Print film produced tighter stars than slide film?

 Uhmmm... :::YES:::. !!!

     While my processing guide is both unfinished and leaning toward the 
artistic over the scientific... most of it is tried and true. Read the first few 
paragraphs here:

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

... also, check your mail from APML yesterday... I explained exposure response 
and the importance of any film reaching it's 'toe'... in this very thread! Here 
it is again, in case you missed it the first time:

----------------------- Forwarded Message -------------------------

     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 !!!

--------------------- End Forwarded Message ----------------------

 Hope This Helps...   :o)

--
 Geoff

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



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