Combining Aperture and Shutter Speed

The combination of aperture (f-number) and shutter speed determines exposure (another important factor in determining exposure is ISO sensitivity, but in the discussion that follows we will assume that ISO sensitivity is fixed). Choosing higher f-numbers correspondingly darkens the image that falls on the image sensor, but you can still achieve optimal exposure if you slow shutter speed in proportion. On the other hand, you can also achieve optimal exposure by choosing a lower f-number and a faster shutter speed. In other words, there are many combinations of aperture and shutter speed that will produce the same exposure. For example, suppose the subject will be correctly exposed at an aperture of f/4 and a shutter speed of 1/250 s. The correct exposure will then also be achieved if you increase the f-number to f/5.6 while slowing shutter speed to 1/125 s. The same is true at f/8 and 1/60 s and at f/11 and 1/30 s.

Combinations of aperture and shutter speed that will produce the same exposure as f/4, 1/250 s

Aperture

F16

F11

F8

F5.6

F4

F2.8

F2

F1.4

Shutter speed (seconds)

1/15

1/30

1/60

1/125

1/250

1/500

1/1000

1/2000

If you always adjust shutter speed to match any changes in aperture, you can achieve correct exposure at any aperture or shutter speed. Note, however, that changing aperture also changes depth of field, while changing shutter speed alters the appearance of moving objects. In other words, you can also adjust aperture for depth of field or shutter speed to produce the effect of motion.

Higher f-numbers, slower shutter speeds

The illustration is an artist’s conception.

Lower f-numbers, faster shutter speeds

The illustration is an artist’s conception.

High f-number, slow shutter speed
The slow shutter speed blurs the jets of water in the fountain; the high f-number increases depth of field, bringing the background into focus.

Low f-number, fast shutter speed
The fast shutter speed appears to stop the water in its tracks; the low f-number reduces depth of field, blurring the background.