question on schwarzschild effect

Hello there

I'm new to this forum and already have a question.
It's about the Schwarzschild effect on long exposure times and I'm not sure whether I got this right. At very weak light, the

photon flux is very low and it can take a very long time until a crystal (within the emulsion) was hit by enough photons to

build up a stable and developable silver core. But it can happen during this time that an already reduced silver atom is reoxidised and thus loosing the effect of the first photon. Is that right so far?

But photons don't come ordered but they are straying randomly around a mean. Thus, it is still possible that a crystal is hit often enough in a sufficently short time to build up a stable silver core. Though it is becoming nonlinearily unlikely (but not impossible) with decreasing photon flux, which might explain the increased exposure time at long exposures (the schwarzschild effect). Is that still right?

But is there an ultimate threshold of lowest possible (but photographable) photon flux? Or is it possible to extend exposure time to hours, days, weeks if neccessary to get a stable latent image? But: there are always photons zipping around, even in practical darkness due to ultra weak photon emission for example. And in case there is no lower limit of the photon flux (and no upper limit of feasible exposure time), wouldn't this mean that over time even a packed film would be exposed? Or is this actually happening and thus a reason for limiting shelf life of undeveloped films?

yours,
thomas

If you call it "reciprocity failure", people are more likely to know what you mean. Yes, your description is about right: you have photons arriving randomly (a Poisson process, to be precise) at each sensitive crystal. A partially-sensitised crystal will return to its base state by leaking the captured electron - also a random process but I think with a narrow distribution, so you can think of it as taking a fixed amount of time to lose the electron.

There is no absolute minimum level because as you noted, photon arrival (from a natural scene, not a controlled source as used in a two-slit experiment) is random. Say you need two photons to form a stable latent image, there is no minimum level of illumination at which you can't by luck get a pair arriving sufficiently close together. Of course, the likelihood drops off rapidly with reducing illumination. Yes, you can make a year-long exposure and there are several decent examples on the web showing building construction, paths taken by the sun throughout the year, etc - typical approach is to use low-sensitivity medium like paper (also has reciprocity failure) and a pinhole.

Films like Acros have excellent reciprocity characteristics because they (mostly) need only a single photon to reach a stable exposed configuration.

Films are sensitive only to photons above a certain energy (proportional to their frequency); longer-wave (redder/colder) photons will not be absorbed by the film. So most thermal photons have no effect. This is the reason why IR films are sensitised no further than 1um - if they were, they would have to be kept refrigerated their whole life and even used in a refrigerated camera with a refrigerated shutter & refrigerated lens. And developed at low temperature, which is even harder.

aha

Hello Polyglot

Thanks for that precise answer. It took me some days to piece this together but couldn't get confirmation. And thanks for the hint with 'reciprocity failure', I mistook it with the german expression.

thomas

I thought this was a joke post.
I was trying to figure out what Arnold schwarzenegger's kids had to do with photography.

mark said:

I thought this was a joke post.
I was trying to figure out what Arnold schwarzenegger's kids had to do with photography.

Click to expand...

Right there with ya, never heard of it referred to the Schwarzchild effect. I was expecting a question regarding how to get away with a illegitimate child....

One more thing that may or may not be obvious from looking at the exposure curve falling off at the bottom end: this effect means you get increased contrast at exposure levels where reciprocity failure is an issue (anything over about 1s for traditional emulsions like Foma & Efke/Adox produce). That's not necessarily a big problem unless you also push the development and get crazy contrast. To make it worse, the falloff curve is convex, which means that you get greater shadow contrast than highlight contrast.

And Schwarzchild effect is a perfectly good name for it, named after its discoverer, it's just that Americans use the term reciprocity failure; the ignorant ones just calling it "reciprocity" which is about as annoying as referring to hypo clearing agent as "hypo". If you look at the Fomapan datasheet, it gives you the reciprocity failure curve (and its one of the worst films for this) labelled as "Schwarzchild effect".

polyglot said:

One more thing that may or may not be obvious from looking at the exposure curve falling off at the bottom end: this effect means you get increased contrast at exposure levels where reciprocity failure is an issue (anything over about 1s for traditional emulsions like Foma & Efke/Adox produce).

Click to expand...

This is not always the case. See: http://www.phototechmag.com/index.php/archive/reciprocity/

There are several good threads on APUG on reciprocity failure, including this one:
(there was a url link here which no longer exists)

Lee

The graph in that page illustrates exactly what I'm saying though only to a tiny extent but that's because it's the graph for the very best films out there in terms of reciprocity performance. If you draw that graph for Fomapan or an Efke/Adox film, you'll see a big sweep upwards. The change in contrast is a function of the fact that as the light intensity reduces, the film's effective speed wrt that light reduces.

Consider a bright-night shot (with 5-stop range, ought to just fit OK in a print) on Fomapan, 2s indicated exposure. Bright parts (Zone VIII) would have a light level implying a 0.25s exposure for Zone V and therefore no reciprocity failure while dark parts (Zone III) would meter at 8s. At that lower light level, a 3-stop reciprocity correction is required, so what should have been nicely textured shadows at Zone III have now dropped off to completely blank (if you don't do the reciprocity-failure correction). If you apply the 3-stop reciprocity correction for the shadows, they're back at Zone III as desired but your midtones (that only required a 1-stop correction) are now up at Zone VII and your highlights are 3 stops off the scale and completely blown. Net effect: get a huge N+3 expansion just from reciprocity failure in a typical scene, without changing development at all.

Solution: don't use old-style film for night shots unless you want to go even longer than indicated and then pull development and cross lots of fingers and toes.

Sure, if you do the test with Acros or TMY2 (edit: or Provia 100F) you will see no difference in contrast at all but they have practically no reciprocity failure whatsoever as evidenced by your link. That is the major reason I love those new films - I can go out at night and just shoot without having to worry about my shadow detail evaporating. And I can do a 10- or 20- minute exposure without having to correct anything (well, maybe half a stop for TMY2) and it will still have the same contrast, whereas an older film would be completely blank.

(this is not me beating on old-style emulsions; I absolutely love Efke-25/CHS-25 but I would never use it for a long exposure)

polyglot said:

Consider a bright-night shot (with 5-stop range, ought to just fit OK in a print) on Fomapan, 2s indicated exposure. Bright parts (Zone VIII) would have a light level implying a 0.25s exposure for Zone V and therefore no reciprocity failure while dark parts (Zone III) would meter at 8s. At that lower light level, a 3-stop reciprocity correction is required, so what should have been nicely textured shadows at Zone III have now dropped off to completely blank (if you don't do the reciprocity-failure correction). If you apply the 3-stop reciprocity correction for the shadows, they're back at Zone III as desired but your midtones (that only required a 1-stop correction) are now up at Zone VII and your highlights are 3 stops off the scale and completely blown. Net effect: get a huge N+3 expansion just from reciprocity failure in a typical scene, without changing development at all.

Click to expand...

Even with exposures at 240 seconds, Bond found no increase in contrast with TMY or Delta 100, no increase in contrast beyond experimental error with Tri-X, and less than a one Zone increase in contrast with HP5+ and TMX. This doesn't include the films with least reciprocity failure like Acros. TMY-2 is decent, but isn't particularly outstanding where reciprocity failure characteristics are concerned.

Have you actually tested Fomapan carefully and gotten the N+3 expansion that you describe? If so, I'd love to see the data. Your logic is correct, but that doesn't appear to be the way current films work in practice, based on the data I've seen from Bond and astrophotographers' testing.

As Bond mentions in his article, most manufacturers' data sheets are woefully out of date and inaccurate where reciprocity failure characteristics are concerned.

FWIW, I actually do sometimes use films with greater reciprocity failure to increase exposure times for things like pinhole work.

Lee

Perhaps its obvious but still not clear to me- if a halfway stable silver core is reoxidised, is this the same compound as before the exposure? Or is it a different, inert, blind compound?

thomas

I believe it reverts to its original, sensitive state. I ain't a chemist though.

As a chemist, I don't know. I think it is possible but unlikely that a crystal could be "desensitized" in a low photon flux. Someone with more knowledge of solid-state physics might know.

As to the term Schwarzschild effect, my first association wasn't to Schwarzenegger (I read German, when I have to), but I did wonder for a moment about the photographic application of black holes.