The Stop
A stop is a factor of two in the light. Every camera control shares this currency.
| Control | Full-stop series | What else it decides |
|---|---|---|
| Aperture | f/1.4 · f/2 · f/2.8 · f/4 · f/5.6 · f/8 · f/11 · f/16 · f/22 | Depth of field |
| Shutter | 1/15 · 1/30 · 1/60 · 1/125 · 1/250 · 1/500 · 1/1000 | How movement records |
| Sensitivity | ISO 50 · 100 · 200 · 400 · 800 · 1600 · 3200 | Grain or noise |
The f-number scale
The f-number is focal length divided by entrance pupil diameter. On a 50 mm lens, f/8 gives a 6.25 mm pupil.
Light passes through an area. Area scales with the square of diameter. To halve the area, divide the diameter by 1.414. This produces the progression: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Because the f-number is a divisor, a larger number means a smaller opening.
The shutter and sensitivity series work similarly. Shutter steps roughly halve time: 15, 30, 60, 125, 250, 500. Sensitivity doubles at each step: ISO 50, 100, 200.
Equivalent exposures
Because all three controls use stops, an adjustment in one balances an adjustment in another.
Suppose the meter reads f/5.6 at 1/60. Changing to f/16 loses three stops (5.6 to 8, 11, 16). The shutter must compensate by three stops: 1/30, 1/15, 1/8.
Exposure is the thing that is conserved; the picture is the thing that changes.
Reciprocity
Exposure equals illuminance multiplied by time. In film, this reciprocity law holds across standard ranges but fails during very long or short exposures.
Common pitfall: ISO does not add light. Aperture and shutter govern light reaching the sensor; sensitivity amplifies the signal, adding noise.