GUIDE G3
Rules of thumb
Exposure without a meter, motion that looks natural, sharp pictures by hand, and light that falls away with distance.
TWICE THE DISTANCE · A QUARTER OF THE LIGHT
- Distance 1ALL OF IT ON ONE SQUARE
- Distance 2SPREAD OVER FOUR SQUARES
Light from a point spreads out: at twice the distance the same light covers four squares instead of one, so each gets a quarter.
Sunny 16
In bright sun, set f/16 and a shutter speed of one over the ISO: at ISO 100, 1/100 or 1/125 s. Then trade stops freely; every line below is the same exposure. For the Moon there is a twin, the Looney 11: f/11 at one over the ISO.
| Aperture | Shutter at ISO 100 |
|---|---|
| f/16 | 1/125 s |
| f/11 | 1/250 s |
| f/8 | 1/500 s |
| f/5.6 | 1/1000 s |
The 180-degree shutter
For video, set the shutter to about one over twice the frame rate: 1/50 s at 25 frames a second, 1/48 or 1/50 at 24. That is a 180-degree shutter, the motion blur we read as normal.
One over the focal length
Without stabilisation, the slowest safe shutter speed by hand is about one over the focal length: 1/60 s with a 50 mm lens, 1/200 s with a 200 mm. Use the full-frame equivalent on a smaller sensor. Stabilisation can buy two stops or more.
Light falls off with the square
Move a small light to twice the distance and the subject gets a quarter of the light, two stops less. It holds for small sources: a bare bulb, a flash.
Try it today
- 01On a sunny day set f/16 and 1/ISO, with no meter.
- 02Check the frame and its histogram.
- 03Open to f/8 and set a shutter four times faster: the same exposure.
- 04Film a few seconds at 1/50 s and at 1/500 s.
- 05Hand-hold your longest lens at one over its focal length, then two stops slower.
Figures from Wikipedia, 'Sunny 16 rule', 'Looney 11 rule', 'Rotary disc shutter', 'Shutter speed', 'Image stabilization' and 'Inverse-square law', read 2026.10.16. Two stops for twice the distance is the quarter the inverse-square law gives, two halvings. The steps under Try it today are working practice, not a standard.