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Why is dust white on negatives and black on reversal?


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Hi there,

I was wondering if someone here might be able to and generous enough to explain (or suggest reading) on the physics of why dust on negatives shows up as white (when printed or scanned to a positive) and black on reversal (or slide still film).

I have known this to be true for a long time but I realized the other day I did not truly understand what was happening. I understand it has to do with the emulsion and processing but that's where my brain hits a bit of a wall. The image that made me realize I didn't get it was one where there was a hair in the gate (black, persistent) and also a white hair on a single frame. I've seen this many times before but not thought much about it.

Here is my logic (It's probably mistaken somewhere but I don't know why!) - On a B&W negative, the light strikes the silver halide and converts it, and in normal processing, the unconverted / unexposed silver halide gets washed away. According to this thought, why wouldn't dust...blocking light, prevent that silver halide conversion just like the hair on the gate does? Is the dust converting the silver halide by directly touching it in some way? What's going on here?

And so then, what's going on with reversal or slide film (probably I need to read up on and grok the E-6 process?)?

The answer doesn't really impact my action, I need to clean the gate and the magazine of course...but..I'm just so curious! Any links to articles or previous answers, or any time spent explaining is most appreciated. Thanks! - Rachel

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The dust isn't usually there at the time of exposure between the light path and the film in gate (though that can happen too), it more often collects on the film roll in the magazine after exposure and at the lab during handling. 

Either way, it usually wouldn't just sit there on the film like glue while it is running through the liquid processing machines (I could be wrong on that part).  But if it did, then it will always be a piece of dust, i.e. opaque. There is no light added in the processing of negative film, the light hitting the film happened in the camera gate and it happens again when striking a print off of the negative.

Light hitting the negative doesn't actually convert silver halide to silver yet, it gives it the potential to be converted later in processing.  Then in the later fixer and wash steps, the remaining unexposed silver halide is removed.

So dust, if it settles on the negative, acts like a piece of density blocking the printer light, and the denser parts of the negative are the brightest parts of the scene, so the dust rephotographs as white on the print (dust later settling on the print would be black.)

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With reversal film, the densities are reversed during processing but since a piece of dust is a piece of density, and on the developed reversal original, now the image is a positive with the denser areas being the shadows, the dust is black.

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Thank you David! We had a big poster up in the darkroom in college that said "Dust is the Enemy!" so maybe that really got into my head ha!

So if I'm understanding you correctly, the dust scenarios in order of likelihood (and these are somewhat theoretical since so far we can't location track a single piece of dust)

1A. dust introduced at the lab and/or printing or scanning stage
1B. long haul dust - gets stuck in the emulsion in the magazine / camera body and hangs out through the liquid development process through printing. (both of these would show up the same in a negative print)
2. Dust present at the moment of exposure but that is perhaps removed in processing before the print/scan - this is probably the theoretical scenario I had in mind when I asked the question...would this theoretical dust show up black on a negative then?

Our darkroom dusting was just stressed in the printing portion. And similarly, now when I scan my own b&w negatives, I rarely encounter a problematic piece of dust that I can't remove physically and scan again (little spots in small quantities are most often left to photoshop)

With motion picture, I'm not in the lab so I know less / have less control so my curiosity here is probably about ranking diligence / effort / concern for motion picture - my prep and film handling, selecting a lab and physical cleaning services, or maybe researching digital removal and then see what's acceptable to me. Thank you for entertaining my curiosity and helping me think it through! I'll go clean my camera now and return to higher order concerns like story, composition and exposure ?

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