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

[Duncan Munro <dmunro@sfu.ca>]

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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