eli5 why are films photos negative?
In: 0
So film is a thin and mostly clear piece of material with a bunch of chemicals that react to light, those chemicals darken when exposed to light.
The more light it is exposed to the darker it gets.
That’s why they are negatives. The dark parts are the parts that got the most light. So when you develop the film that becomes the brightest part.
Photographic film is limited by the chemicals we can find that are light sensitive and can react in a way to form an image. One of the earlier and most popular class of chemicals used are silver halides.
It turns out that silver halides, on exposure to light, can turn into metallic silver and elemental halogen (often starting with silver bromide, so elemental bromine). At the microscopic level, embedded in the film gelatin, these tiny silver crystals appear black. So it’s the fact that the chemical exposed to light turns black and the chemical that isn’t exposed to light doesn’t, plus the latter can be washed away, leaving the metallic silver behind, that results in an image that’s the opposite of the light exposure.
Colors are more complicated, but some work by having other chemicals or dyes of the appropriate color attached to the silver halide, influencing what color light it’s sensitive to or what color it appears as. But to the extent it’s based on silver halide technology, it will still be that the part exposed to the most light will be darkest.
As u/paulmarchant points out, color slide film works differently. There were, in fact, multiple chemical systems for color photography, some of which are still available.
For slide film, the developed image is a direct positive so that it can be projected straight from that original piece of film onto a screen.
But where the end result is intended to be printed on paper, it makes more sense for the developed image on the film to be a negative so the image can then be projected (using an “enlarger”) onto light-sensitive paper. This paper is initially white and responds to light falling on it by going darker. This means that light (i.e. transparent) regions on the negative will let lots of light pass through and be projected onto the paper, making those projected regions go darker on the paper. Darker regions on the negative will tend to block the light so less light will hit the paper and that region won’t be darkened by the paper’s reaction to light, so that region in the image on paper will stay lighter. In this way, the negative image on the developed film becomes a positive image on the paper.
Latest Answers