Reciprocity Failure

Beyond a certain length of exposure, film no longer absorts light in a linear fashion, requiring longer and longer times to achieve the equivalent exposure. This is known as reciprocity failure, and is important when working with long exposures, such as with pinhole cameras. Here’s a brief guide to some popular films and their reciprocity characteristics.

Kodak Ektar
Up to approximately 10 seconds, there is no significant exposure discrepancy. Astrophotographers may also be interested to learn that unlike almost all other color negative films, Kodak Ektar 100 film has strong response to hydrogen-alpha.
 2 sec = 2 sec
4 sec = 4 sec
8 sec = 8 sec
16 sec = 20 sec
32 sec = 40 sec
64 sec = 115 sec
2 m 18 s = 4 m 16 s
4 m 16 s = 9 m 23 s
Kodak Ektar 100 Data Sheet

Kodak T-Max
A 1 second exposure requires +1/3 stop, 10 seconds requires +1/2 stop (or exposure for 15 seconds instead of 10) and a 100 second exposure requires +1 stop (or expose for 200 seconds instead of 100).
Kodak T-Max Data Sheet

Kodak Tri-X Pan
1 min. = 8 mins.
Kodak Tri-X Data Sheet

Kodak Plus-X
1 second +1 stop, 10 seconds +2 stops, 100 seconds +3 stops.
Kodak Plus-X Data Sheet

Kodak Portra 160, 400
No compensation required up to 10 seconds.

Kodak Porta 160 Data Sheet

Kodak Porta 400 Data Sheet

Kodak Porta 800
No compensation required up to 1 second.
Kodak Porta Films Data Sheet

Kodak Ultra Max 400
No data
Kodak Ultra Max 400 Data Sheet

Kodak Ultra Max 800
No data
Kodak Ultra Max 800 Data Sheet

Kodak High Definition 400
No data

Kodak High Definition 400 Film Data Sheet

Kodak Gold 100/200

No data

Kodak Gold 100/200 Data Sheet

Kodak BW400CN

No data

Kodak BW400CN Data Sheet

Kodak Advantix

No data

Kodak Advantix Films Data Sheet

Kodak Ektachrome 64T (tungsten)

Designed for long exposures and has excellent reciprocity.
Kodak Ektachrome 64T Data Sheet

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Fujifilm Neopan 100 Acros
Up to 128 second (2 minutes) without compensation. For exposure times of 2 minutes to around 15 or 16 minutes, Fuji recommends +1/2 stop.
Fujifilm Neopan 100 Acros Data Sheet

Fujifilm Provia
Up to 128 seconds without compensation.
Fujifilm Provia 100F Data Sheet

Fujifilm Astia
Similar to Provia, but with less color shift even up to 20 minutes.
Fujifilm Astia 100F Data Sheet

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Agfa 25 or Rollei Pan 25
1 second is +1 stop, 10 seconds is +1.5 stops, 100 seconds is +2 stops.
Rollei Pan 25 Data Sheet

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Efke PL25
to 1/2 second - 0 stop
1 second - 1/3 stop
10 seconds - 2/3 stop
100 seconds - 1.5 stops