Photography
Controlling light, and what each control costs.
Photography records an image by exposing a light-sensitive surface. Three settings control the exposure, and each has a side effect beyond brightness — which is what makes photography a set of trade-offs rather than a set of correct values.
The exposure triangle
| Control | Governs | Side effect |
|---|---|---|
| Aperture (f-number) | How wide the lens opens | Depth of field. Low f-number = wide opening = shallow focus |
| Shutter speed | How long the sensor is exposed | Motion. Fast freezes, slow blurs |
| ISO | Sensitivity to light | Noise. Higher ISO, grainier image |
The counter-intuitive part is the f-number: it is a ratio, so a smaller number means a wider aperture. f/1.8 lets in far more light than f/16 and gives much shallower depth of field.
Trade-offs in practice
Freezing a sprinter needs a fast shutter, which means less light, so the aperture must open or the ISO rise — buying sharpness with either shallow focus or noise. A landscape wanting everything sharp needs a narrow aperture, so the shutter must stay open longer, which needs a tripod. There is no setting that avoids the choice.
Composition
The rule of thirds places subjects on the lines of a 3×3 grid rather than dead centre. Leading lines direct the eye. Framing uses foreground elements to enclose the subject. Fill the frame to remove distraction. These are defaults worth knowing precisely so that departing from them is a decision.