Metering Fundamentals
A reflected-light meter measures light bouncing off the subject. It assumes every scene is a mid grey. Therefore, it turns white snow dark and black coal pale.
Conventionally, that target is an 18 per cent reflectance card. Different camera makers calibrate slightly below this value, causing minor discrepancies.
An incident-light meter points from the subject toward the camera. It measures light falling onto the scene. Because subject reflectance is ignored, tones render accurately.
| Method | Measures | Fooled by |
|---|---|---|
| Reflected, averaging | Whole frame | Scenes far from average reflectance |
| Reflected, centre-weighted | Mostly the centre | Backlight or bright sky |
| Reflected, spot | One small area | User error in target choice |
| Incident | Light falling on subject | Meter lit differently from subject |
Common pitfall: assuming a spot meter is automatically more accurate. It only tells you how to make a tiny patch mid grey.
Exposure Rules and Flash
Exposure Value (EV) combines settings into a single figure at ISO 100:
- Bright sun: EV 15
- Heavy overcast: EV 12
- Lit street at night: EV 8
- Living room: EV 6
Sunny 16 provides EV 15 directly. In bright sun, set f/16 and match shutter speed to ISO:
- ISO 100: f/16 at 1/125
- ISO 200: f/16 at 1/250
Flash Calculations
A flash's guide number equals distance multiplied by the correct f-number.
- A guide number of 32 at ISO 100 gives f/8 at 4 metres ().
This relies on the inverse-square law. Light spreads over a sphere, so intensity falls with the square of distance. Doubling distance cuts the light to one quarter, losing two stops.