Past Date

Film loses sensitivity and gains base fog as it ages. Fog is a uniform veil of density that swallows shadow detail, flattens contrast and, in colour film, shifts colours because each dye layer ages at a different rate. Extra exposure lifts your shadows above that fog.

The app works out an "effective age" from:

• Years past date. The well-known starting point is about 1 stop per decade.
• Storage. Frozen film ages about 5× slower than film kept at room temperature, and fridge storage about 2× slower. Lofts, cars and hot climates speed ageing up a lot.
• Speed. Fast film (800+) fogs much faster, partly from background radiation that cold storage can't stop.
• Emulsion. Professional stocks tend to hold up a little better than consumer ones.

Colour negative gets about 1 stop of extra exposure per effective decade, because it handles overexposure easily. Black and white gets slightly less. Slide film mostly gains fog and colour shifts rather than losing speed, and overexposure blows its highlights, so it gets only a small correction.

The preview is an illustration built from these numbers, not a lab scan. Real results vary a lot from roll to roll, even within one batch. Photographers disagree about the "stop per decade" rule, which is why the app suggests bracketing whenever storage is uncertain.

Based on: EMULSIVE, Old Cams by Jens, Fstoppers, Silberra / Silver Grain Classics, and Photrio forum discussions on benzotriazole and expired slide film.