Exposure, Metering and Colour Balance
This section covers Exposure, Metering and Colour Balance: the exposure triangle, meter behaviour, exposure value, histograms, white balance and colour casts under different sources.
Core rules
- Exposure triangle: aperture, shutter speed and ISO. One stop equals halving or doubling light or sensitivity.
- Reflected meters assume an average scene reflects about 18% grey. Bright snow or a white wall makes the meter underexpose; dark coal makes it overexpose.
- Metering modes: evaluative or matrix, centre-weighted, and spot. Incident metering reads light falling on the subject and avoids subject-reflectance bias.
- Exposure compensation: add light for bright scenes, subtract for dark scenes. Bracket when uncertain.
- Histogram: left is shadows, right is highlights. Clipping at either end loses detail; a colour-channel clip can occur while luminance looks safe.
- White balance neutralises the colour temperature of the source. Presets include tungsten about 3200 K, daylight about 5500 K and shade about 7000 K. A grey card gives a reference.
- Mixed lighting produces different casts in different areas; correct the dominant source or gel the lights.
Worked example
A snowfield fills the frame and the meter recommends 1/500 s at f/8. The snow will record grey, not white. Reasoning: the meter wants 18% grey; snow is much brighter. Add about +1.5 to +2 EV: use 1/125 s at f/8, two stops more light, or f/4 at 1/500 s. Check the histogram so highlights sit near but not beyond the right edge. If a person is in the frame, spot-meter the face and expose for skin tone instead.
Traps
- Using white balance as an exposure control.
- Trusting a single histogram channel; red clipping can hide in a normal luminance histogram.
- Metering a black object and then subtracting exposure, making it blacker and losing detail.
- Forgetting reciprocity failure with long film exposures.