Re: [APML] Scanning slides (Was IC 410 uploaded and IC 405 adjusted)

["Stuart Heggie" <stuart.j.heggie@sympatico.ca>]

Duncan, you have described more fully what I was trying to say. Thanks for that! In Robert Reeves'
book section 7.15 it explains as follows (talking about B&W film but I think it is the same thing
with colour):

"Until enough photon-events cause a given crystal to produce at least one group of about four or
five neutral silver atoms in one place, the incipient latent image is unstable, and can thermally
decay even as it is building up."

So, per you description below, the name for this is Reciprocity Law Failure (or Reciprocity Failure
for short). If the system is slow enough, the crystals being formed will keep decaying because of
insufficient photons in a short enough time to stabilize the crystallization.

Now, as John Mirtle has pointed out, if Alan is getting different results from others with the SAME
optical configuration and film, this cannot be the explanation.

Alan, I think your idea of a processing error might be right or maybe others are push processing
their film and you are not?

Stuart
----- Original Message -----
From: "Duncan Munro" <dmunro@sfu.ca>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Thursday, November 04, 2004 12:42 AM
Subject: Re: [APML] Scanning slides (Was IC 410 uploaded and IC 405 adjusted)


> I think you are talking about reciprocity failure. All films suffer from
> this to some extent. Basically, it means that the longer your exposure
> time the less sensitive the films becomes.
>
> Ideally film density will increase in a linear fashion. IE twice the
> exposure time will compensate for a reduction in illumination by 1/2.
> However, past a certain minimum illumination a further reduction in
> illumination by 1/2 (for example) might result in the need to triple
> rather than double the exposure time.  This results from a variety of
> factors but the main culprit is that the individual grains in the
> emulsion do have a minimum theshold of illumination below which they
> will not respond in a linear fashion.
>
> One the reasons for hypering film, especially tech pan, is than it
> decreases the tendency of film to suffer form reciprocity failure.
> Certain films, such as E-200 have a higher resistance to reciprocity
> failure than other films which is way they are favoured for
> astrophotography. This is why E-200 can record more density in long
> exposures than other film with a faster nominal ISO rating. However,
> I'm sure that E-200 will also begin to suffer greatly from reciprocity
> failure as exposure times increase. Basically reciprocity failure could
> be called "the law of diminishing returns"
>
> cheers
>
> Duncan
>
> Alan Voetsch wrote:
>
> >Hey Stuart,
> >
> >--- Stuart Heggie <stuart.j.heggie@sympatico.ca> wrote:
> >
> >
> >>emulsion that may be a factor is this: Each grain has to be hit by
> >>more than a certain number of
> >>photons or the chemical reaction reverses (so I'm told). This
> >>suggests that at sufficiently high
> >>f-ratio and hence exposure time, the number of photons hitting the
> >>grain in the emulsion is too few
> >>over any time frame to sustain the chemical reaction and so you are
> >>losing signal.
> >>
> >>
> >
> > I understand, but have never heard this. That doesn't mean it isn't
> >so. And if true, would explain a problem. Then again, others would also
> >have the same result, at least those who are using a similar setup.
> >Let's see.
> >
> >To the film experts: can the chemical reaction reverse if not hit by
> >enough photons?
> >
> >Thanks,
> >Alan
> >
> >
> >
> >
> >=====
> >SCT Astrophotography: http://www.pbase.com/avoetsch/astrophotography
> >FS-102 G-11/Gemini: http://www.pbase.com/avoetsch12952/tak_fs102
> >& http://www.pbase.com/avoetsch12952/fs102
> >
> >
> >
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