The Exposure Triangle
曝光三角 — 胶片摄影的光圈、快门与 ISO
The exposure triangle is the fundamental concept in photography: three interdependent settings — aperture, shutter speed, and ISO — that together determine the exposure of your photograph.
The Three Elements / 三大要素
1. Aperture (光圈)
Aperture is the size of the lens opening, measured in f-stops (f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22).
- Larger opening (small f-number like f/1.4) = more light, shallower depth of field
- Smaller opening (large f-number like f/16) = less light, deeper depth of field
Each full stop doubles or halves the light: f/1.4 → f/2 → f/2.8 → f/4 → f/5.6 → f/8 → f/11 → f/16 → f/22
光圈控制镜头进光量的大小。f 值越小（如 f/1.4），进光越多、景深越浅；f 值越大（如 f/16），进光越少、景深越深。每相邻一档光圈，进光量相差一倍。
2. Shutter Speed (快门速度)
Shutter speed is how long the shutter stays open, measured in fractions of a second or full seconds.
- Fast shutter (1/1000s) = freezes motion, less light
- Slow shutter (1/30s, 1s, 30s) = motion blur, more light
Standard full-stop scale: 1/1000 → 1/500 → 1/250 → 1/125 → 1/60 → 1/30 → 1/15 → 1/8 → 1/4 → 1/2 → 1s → 2s → 4s …
快门速度控制感光材料的曝光时间。快门越快（如 1/1000s），可以冻结运动、进光量少；快门越慢（如 1s），会产生运动模糊、进光量多。
3. ISO (感光度)
ISO determines the film’s sensitivity to light. In film photography, ISO is fixed for the entire roll — you choose it when you load the film.
- Low ISO (50, 100) = fine grain, less sensitive, needs more light
- High ISO (400, 800) = more grain, more sensitive, works in lower light
Common film ISOs: 50, 100, 160, 200, 400, 800
ISO 感光度决定胶片对光线的敏感程度。低 ISO（50/100）颗粒细腻但需要更多光线；高 ISO（400/800）颗粒较粗但适合弱光环境。与数码不同，胶片的 ISO 在装入后就固定了。
How They Work Together / 三者如何配合
The exposure triangle is a balancing act: changing one setting requires adjusting another to maintain the same exposure.
| Scenario | Aperture | Shutter | ISO | Result |
|---|---|---|---|---|
| Bright daylight portrait | f/2.8 | 1/500 | 400 | Shallow DOF, frozen subject |
| Landscape in golden hour | f/11 | 1/60 | 100 | Deep DOF, tripod recommended |
| Street photography | f/8 | 1/250 | 400 | Moderate DOF, decisive moments |
| Night long exposure | f/8 | 30s | 100 | Deep DOF, light trails |
| Indoor available light | f/1.4 | 1/30 | 800 | Very shallow DOF, handheld limit |
The Sunny 16 Rule
A classic film photography rule of thumb: on a sunny day, set aperture to f/16 and shutter speed to 1/ISO.
- ISO 400 film on a sunny day: f/16 at 1/500s (nearest standard speed)
- ISO 100 film on a sunny day: f/16 at 1/125s
阳光十六法则：晴天时，光圈设为 f/16，快门设为 1/ISO。例如 ISO 400 的胶片在晴天：f/16、1/500s。这是胶片摄影中最经典的快速测光法则。
Film ISO vs Digital ISO / 胶片 ISO 与数码 ISO 的区别
| Aspect | Film ISO | Digital ISO |
|---|---|---|
| Changeable? | Fixed per roll | Adjustable per shot |
| Grain/Noise | Silver halide grain (organic) | Electronic noise (digital) |
| High ISO look | Visible grain, retained detail | Noise, potential detail loss |
| Push/Pull | Possible in development | N/A — adjusted in-camera |
| Latitude | Negative film: 2-3 stops overexposure tolerance | Limited highlight recovery |
Push and Pull Processing
Unlike digital, film photographers can intentionally rate film at a different ISO and compensate during development:
- Push processing: Rate ISO 400 film at ISO 800 or 1600, then extend development time. Increases contrast and grain.
- Pull processing: Rate ISO 400 film at ISO 200, then reduce development time. Decreases contrast.
推冲与拉冲：胶片摄影独有的技术。推冲（push）= 按更高 ISO 拍摄，延长冲洗时间；拉冲（pull）= 按更低 ISO 拍摄，缩短冲洗时间。推冲增加对比度和颗粒，拉冲则相反。
Stop Increments / 档位增量
FilmMeter supports three step sizes for precise exposure control:
| Step Size | Aperture Example | Shutter Example |
|---|---|---|
| Full stop | f/2.8 → f/4 | 1/125 → 1/250 |
| 1/2 stop | f/2.8 → f/3.5 → f/4 | 1/125 → 1/180 → 1/250 |
| 1/3 stop | f/2.8 → f/3.2 → f/3.5 → f/4 | 1/125 → 1/160 → 1/200 → 1/250 |
Most modern cameras use 1/3-stop increments. Older manual cameras typically use full stops or half stops.
FilmMeter 支持三种步进精度：整档、半档、1/3 档。现代相机多用 1/3 档，老式手动相机通常为整档或半档。选择与你的相机匹配的步进精度可以获得最精确的曝光设置。
Exposure Compensation / 曝光补偿
Sometimes the meter reading needs adjustment based on the scene:
| Scene | Compensation | Reason |
|---|---|---|
| Snow / white sand | +1 to +2 EV | Meter wants to underexpose white |
| Black subject | -1 to -2 EV | Meter wants to overexpose dark |
| Backlit subject | +1 to +2 EV | Background is much brighter |
| Stage lighting | -1 EV | High contrast, preserve highlights |
FilmMeter’s AI Scene Detection handles this automatically by recognizing scene types and applying appropriate compensation.
测光表总是试图将场景还原为中灰（18% 灰）。雪景、白沙需要增加曝光（+1~2 EV），深色主体需要减少曝光（-1~2 EV）。FilmMeter 的 AI 场景识别可以自动处理这些补偿。
Practicing with FilmMeter / 用 FilmMeter 实践
FilmMeter is the ideal tool for learning and applying the exposure triangle:
- Real-time metering — See how aperture, shutter speed, and ISO interact as you adjust each setting
- Step size control — Switch between 1/3, 1/2, and full stop increments to match your camera
- Scene detection — AI automatically suggests exposure compensation for tricky scenes
- Depth of field preview — See the DOF impact of your aperture choice with our DOF calculator
- Film stock selection — Choose from 21 film stocks and see how ISO affects your available settings