Film ISO Light Speed Relation

Film ISO, Light Speed & Exposure Relationship

ISO tells you how sensitive the film is to light. On the modern arithmetic scale, every stop doubles the number: ISO 100 → 200 → 400 → 800. Each time it doubles you can cut your exposure time in half or close the aperture by one f-stop. The old ASA scale ran on the same numbers; DIN, on the other hand, was logarithmic, so ISO 400 came out as DIN 27°.

Low-ISO emulsions like Kodak Ektar 100 or Velvia 100 give fine grain and saturated color, perfect for tripod landscape work. ISO 400 is the versatile sweet spot — Kodak Tri-X 400, Ilford HP5+ and Portra 400 handle handheld street shooting in mixed light. ISO 800–1600 stocks (Portra 800, Delta 3200, Cinestill 800T) and pushed film (Tri-X @ 1600) accept low light at the cost of higher contrast and visible graininess. The exposure formula is EV = log₂(N²/t) − log₂(ISO/100).

Applications

Useful for converting light-meter readings between films, planning push/pull processing, matching shutter speed to a new ISO when light changes, and teaching the reciprocity of ISO, aperture and time.

FAQ

What does "pushing" ISO mean? Shooting a film at higher ISO than its box speed (e.g. HP5 @ 1600) and compensating with extended development time — trades shadow detail for usable midtones in dim light.

Is ISO the same as ASA? Numerically yes — ISO 400 = ASA 400. The ISO standard simply combined ASA (arithmetic) and DIN (logarithmic) into one notation in 1974.

Why does high ISO grain look harsher than digital noise? Film grain is physical silver halide clumps; their size grows with sensitivity. Push processing amplifies them further, producing the gritty look prized in reportage photography.