Gradual progression (au fur et à mesure que)

Gradual Progression

To talk about two actions that progress simultaneously or proportionally, use the phrase au fur et à mesure que (as / gradually as).

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

Because this expression describes factual, ongoing events in real time, it is always followed by the indicative mood (standard tenses like the present).

You improve the sharpness as you enlarge the photo.
Tu améliores la netteté au fur et à mesure que tu agrandis la photo.
The contrast appears as you agitate the chemistry.
Le contraste apparaît au fur et à mesure que tu agites la chimie.

Use it to connect two clauses where one ongoing action paces the other—perfect for step-by-step processes like working with photography chemicals.