Learn French: La pellicule change au fur et à mesure que le révélateur agit. - The roll of film changes as the developer acts.

Au fur et à mesure que emphasizes progression and proportion. It doesn't just mean the film changes at the same time the developer acts, which pendant que would imply; it means the film changes gradually in direct proportion to the developer's ongoing action.

When do I use au fur et à mesure que instead of au fur et à mesure de?

You use que when it introduces a full clause with a conjugated verb, like le révélateur agit in this sentence. You use de when the phrase is followed by a noun, for example: au fur et à mesure de l'action (as the action progresses).

Why is the verb agit in the indicative mood rather than the subjunctive?

Even though many conjunctions ending in que trigger the subjunctive, au fur et à mesure que states a factual, ongoing reality rather than a doubt, desire, or possibility. Because it describes something actually happening, it always takes the indicative mood.

Why use pellicule instead of film for "film"?

In French, un film usually refers to a movie you watch on a screen. Une pellicule refers to the physical roll of photographic film used in cameras. It can also refer to any thin layer or coating, and is even the French word for dandruff!