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