Reciprocity Failure Guide
倒易率失效 — 胶片长曝光补偿完全指南
When film exposures exceed approximately 1 second, the relationship between light intensity and exposure time becomes non-linear. This phenomenon is called reciprocity failure (or reciprocity law failure), and it requires exposure compensation to achieve correct density on film.
What is Reciprocity Failure? / 什么是倒易率失效？
The reciprocity law states that exposure = intensity × time. For most photography, doubling the time at half the intensity produces the same result. However, at very long or very short exposure times, film emulsion becomes less sensitive — this is reciprocity failure.
倒易率（互易律）指的是曝光量 = 光强 × 时间。在大多数情况下，将时间加倍、光强减半可以得到相同的曝光效果。但在极长或极短曝光时间下，胶片乳剂的感光效率会降低——这就是倒易率失效。
Why It Matters for Film Photographers
- Night photography: Exposures of 10 seconds or more require significant compensation
- Long exposure landscapes: Waterfalls, cloud streaks, star trails
- Low-light interiors: Architectural and interior photography
- ND filter use: When neutral density filters push exposure times beyond 1 second
Compensation Methods / 补偿方法
Method 1: Power Formula (Tc = Tm^P)
The most common mathematical approach uses a power factor P specific to each film stock:
Corrected Time = (Metered Time) ^ P
Where:
- Tc = corrected exposure time (seconds)
- Tm = metered exposure time (seconds)
- P = reciprocity factor (film-specific, typically 1.1 to 1.5)
Example: If your meter reads 4 seconds with Kodak Portra 400 (P ≈ 1.3):
- Tc = 4^1.3 ≈ 6.1 seconds
公式法：校正时间 = 测量时间^P。其中 P 是每种胶片特有的倒易率系数，通常在 1.1 到 1.5 之间。P 值越大，该胶片在长曝光时需要的补偿越多。
Method 2: Lookup Tables
Some manufacturers provide specific lookup tables mapping metered time to corrected time. This is more accurate than the formula method when official data is available.
| Metered Time | Example Correction (P=1.3) |
|---|---|
| 1s | 1.0s |
| 2s | 2.5s |
| 4s | 6.1s |
| 8s | 15.0s |
| 15s | 33.6s |
| 30s | 82.4s (≈ 1m 22s) |
| 60s | 202s (≈ 3m 22s) |
| 120s | 496s (≈ 8m 16s) |
Film Stock Reciprocity Data Summary / 各胶卷倒易率数据
The table below summarizes reciprocity characteristics for all 21 film stocks in the FilmMeter database. Click each film name for detailed specifications.
Color Negative Films / 彩色负片
| Film Stock | ISO | P Factor | Threshold | Data Source |
|---|---|---|---|---|
| Kodak Portra 400 | 400 | 1.30 | 1s | Kodak Technical Pub |
| Kodak Portra 160 | 160 | 1.30 | 1s | Kodak Technical Pub |
| Kodak Portra 800 | 800 | 1.30 | 1s | Kodak Technical Pub |
| Kodak Gold 200 | 200 | 1.30 | 1s | Kodak Technical Pub |
| Kodak ColorPlus 200 | 200 | 1.30 | 1s | Estimated |
| Kodak UltraMax 400 | 400 | 1.30 | 1s | Estimated |
| Kodak Ektar 100 | 100 | 1.30 | 1s | Kodak Technical Pub |
| Fujifilm Superia 400 | 400 | 1.30 | 1s | Estimated |
| CineStill 800T | 800 | 1.30 | 1s | Community tested |
| Harman Phoenix 200 | 200 | 1.35 | 1s | Estimated |
| LeKai C200 | 200 | 1.30 | 1s | Estimated |
| Kodak 5207 250D | 250 | 1.30 | 1s | Kodak Technical Pub |
| Kodak 5219 500T | 500 | 1.30 | 1s | Kodak Technical Pub |
Black & White Films / 黑白胶片
| Film Stock | ISO | P Factor | Threshold | Data Source |
|---|---|---|---|---|
| Ilford HP5 Plus | 400 | 1.31 | 1s | Ilford Official |
| Ilford Delta 400 | 400 | 1.41 | 1s | Ilford Official |
| Ilford Delta 100 | 100 | 1.26 | 1s | Ilford Official |
Reversal / Slide Films / 反转片
| Film Stock | ISO | P Factor | Threshold | Data Source |
|---|---|---|---|---|
| Fujifilm Velvia 50 | 50 | 1.30 | 1s | Fujifilm Technical |
| Fujifilm Velvia 100F | 100 | 1.30 | 4s | Fujifilm Technical |
| Fujifilm Provia 100F | 100 | 1.30 | 1s | Fujifilm Technical |
| Kodak Ektachrome E100 | 100 | 1.30 | 1s | Kodak Technical Pub |
| Kodak 5294 100D | 100 | 1.30 | 1s | Estimated |
Film Type Differences / 不同胶片类型的差异
Black & White
B&W films generally have well-documented reciprocity data. Ilford provides official reciprocity correction factors for all their films. B&W films are also more forgiving due to wider exposure latitude.
Color Negative
Modern color negative films like Kodak Portra have relatively mild reciprocity failure. Kodak publishes some data in technical publications, but many consumer films rely on community-tested estimates.
Reversal / Slide Film
Slide films have the narrowest exposure latitude, making accurate reciprocity compensation critical. Slight underexposure from uncorrected reciprocity failure can result in noticeably dark slides.
黑白片通常有最完整的官方数据，且宽容度高，容错空间大。彩色负片现代产品倒易率失效较轻。反转片宽容度最窄，倒易率补偿的准确性至关重要。
Practical Tips / 实用技巧
- Start compensating at 1 second — Most films begin showing reciprocity failure around 1s exposure time
- Bracket your exposures — When in doubt, shoot one frame at calculated compensation and one at +1 stop
- Consider color shifts — Long exposures on color film can cause color shifts; this is more pronounced on slide film
- Use FilmMeter’s auto compensation — The app automatically calculates corrected exposure times based on your selected film stock
实用建议
- 1秒以上开始补偿 — 大多数胶片在1秒以上开始出现明显的倒易率失效
- 包围曝光 — 不确定时，按计算值拍一张，再多加一档拍一张
- 注意色偏 — 彩色胶片长曝光会产生色偏，反转片更为明显
- 使用 FilmMeter 自动补偿 — App 会根据所选胶卷自动计算校正后的曝光时间
Automatic Compensation with FilmMeter / 使用 FilmMeter 自动补偿
FilmMeter includes built-in reciprocity failure compensation for all 21 film stocks. When your metered exposure exceeds the threshold (typically 1 second), the app automatically:
- Detects that the exposure requires compensation
- Applies the film-specific P factor or lookup table
- Displays both the metered and corrected exposure times
- Lets you choose your preferred correction method