A photograph is a quantity of light, collected. There are only two ways to change how much of it arrives: make the hole in the lens bigger, or hold that hole open for longer. The camera calls them aperture and shutter speed. They can stand in for each other exactly, which is the useful part, and neither of them does only that one job, which is the part that takes a while to learn.
A stop is a doubling
Before either control makes sense, the unit does. Photographers count light in stops, and one stop is a doubling or a halving of it. Two stops is four times the light. Three stops is eight. It is a scale of ratios rather than amounts, because that is how the eye and the sensor both behave: what matters is not how many photons arrived, but how many arrived compared with the last picture.
Aperture is counted in stops. Shutter speed is counted in stops. ISO is counted in stops. That shared unit is the whole reason the three can be traded against one another, and it is why a photographer will ask for another stop without saying which dial to turn.
Aperture: the size of the hole
Inside the lens sits a ring of overlapping blades called the diaphragm. The opening they leave between them is the aperture, and it is written as an f-number: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Each step along that scale halves the light of the step before it.
The numbers run backwards to intuition, and this is the most common early stumble. f/1.4 is a wide hole passing a great deal of light. f/22 is nearly shut. The reason is that an f-number is a ratio, the focal length divided by the diameter of the opening, so a larger number means the opening is a smaller fraction of the lens.
Drag the dial and watch the blades. The orange disc is the light getting through, and it does not shrink gently: every notch is half of what it was.
What aperture costs: depth of field
If aperture only changed brightness you would leave it wide open and forget it. It also decides how much of the scene is in focus, which is called depth of field, and that is usually the real reason to choose one f-number over another.
Wide open, the sharp zone is a thin slice. On an 85mm lens at f/1.8, focused two metres away, that slice is around four centimetres deep, which is not enough to hold both of a person’s eyes if they turn their head. That thinness is exactly why portraits are taken there: the face separates from the wall behind it, and the background dissolves into tone.
Stopped down to f/11 or f/16, the sharp zone reaches from the foreground to the horizon, which is why landscapes live at the other end of the scale. Most lenses are also at their sharpest in the middle, around f/8, where the glass has stopped showing its flaws and diffraction has not yet begun to soften the picture.
Shutter speed: the length of the look
The shutter is a curtain in front of the sensor, and shutter speed is how long it stays out of the way: 1/1000 of a second, 1/60, four whole seconds. The scale behaves exactly like the aperture scale. 1/500 is half the light of 1/250, which is half the light of 1/125. One notch, one stop.
Press the button in the diagram. The ball is crossing the frame the whole time; what the picture records is wherever the ball was for as long as the shutter was open. At a fast speed it is a ball. At a slow one it is a stripe, because it genuinely was in all of those places while the sensor was looking.
What shutter speed costs: movement
A number worth knowing for each kind of subject. Around 1/250 to 1/500 holds a walking person still. Sport wants 1/1000 to 1/2000. A bird in flight wants 1/2000 or faster. Below about 1/60, anything alive begins to smudge.
Then there is your own body. The old rule is that a handheld shutter should be faster than one over the focal length: 1/200 on a 200mm lens, 1/50 on a 50mm. On a crop-sensor camera, multiply the focal length by the crop factor first, so that 200mm on a 1.5x body wants 1/300. Stabilisation buys back a few stops of this, but it steadies the camera, not the subject.
Blur is not only a failure, though. Swing the camera along with a passing cyclist at 1/30 and you get a sharp cyclist against a streaked street, which says fast far better than a frozen frame does. Long exposures turn a waterfall into silk and a crowd into fog. Slow is a perfectly good choice, as long as it is a choice.
The trade: equivalent exposure
Here is the rule the whole thing rests on, called reciprocity. Open the aperture by one stop and halve the shutter time, and the sensor receives exactly the same light. f/8 at 1/125 is the same exposure as f/5.6 at 1/250, and the same as f/11 at 1/60. Three different photographs, one brightness.
Drag the slider. The orange square in the middle is the light landing on the sensor, and it never changes: whatever you take from the hole you hand back to the clock. What does change is what the picture is about. f/2 at 1/1000 is a sharp face against a soft background. f/16 at 1/15 is everything sharp and everything moving smeared.
So which do you set first?
Whichever one the photograph is about. Then let the other follow.
If the picture is about movement, set the shutter first: choose the speed that freezes or smears the thing you care about, and let the aperture land where it must. That is what shutter priority does, marked S or Tv on the dial. If the picture is about what is sharp and what dissolves, set the aperture first: choose the depth of field and let the shutter follow. That is aperture priority, marked A or Av, and it is where most photographers spend most of their lives.
The camera will gladly choose both for you. It will choose the safe middle every time, and the safe middle is how you end up with a correctly exposed photograph of nothing in particular.
The third dial
All of the above assumes a fixed sensitivity. When the light runs out and both controls are already at their limits, ISO buys light out of nothing and pays for it in grain. It moves in stops like the other two, so it joins the same trade. Together the three are called the exposure triangle, and it deserves its own piece.
Where to start, by subject
There is no law, and light does not read charts. But most subjects have a pair of settings photographers reach for first and then adjust from, and knowing them saves a great deal of fumbling. Set the one printed in bold and let the camera work out the other.
| Subject | Aperture | Shutter | Set first |
|---|---|---|---|
| Portrait, one person | f/1.8 to f/2.8 | 1/125 or faster | Aperture |
| Group of people | f/5.6 to f/8 | 1/125 or faster | Aperture |
| Landscape | f/8 to f/16 | whatever it takes, tripod | Aperture |
| Street | f/8 | 1/250 | Either |
| Wildlife | f/4 to f/5.6 | 1/1000 to 1/2000 | Shutter |
| Birds in flight | f/5.6 | 1/2000 to 1/4000 | Shutter |
| Sport, indoors | f/2.8, wide as it goes | 1/800 to 1/1000 | Shutter |
| Sport, outdoors | f/4 to f/5.6 | 1/1000 to 1/2000 | Shutter |
| Panning a moving vehicle | f/8 to f/11 | 1/30 to 1/60 | Shutter |
| Waterfall, silk water | f/11 to f/16 | 1/2 to 2 seconds, tripod | Shutter |
| Indoors, no flash | f/1.8 to f/2.8 | 1/60 to 1/125 | Aperture |
| Night street | f/2.8 to f/4 | 1/60, ISO up | Aperture |
| Stars | f/2.8, wide as it goes | 15 to 25 seconds, tripod | Both, by hand |
Two things travel with every row. Handheld, keep the shutter faster than one over the focal length, whatever the table says. And when the light will not give you both numbers, ISO is what pays the difference.
The short version
- One stop is double or half the light, and both controls move in stops.
- Small f-number, wide hole, shallow focus. Large f-number, small hole, deep focus.
- Fast shutter freezes, slow shutter smears. Handheld, stay faster than one over the focal length.
- A stop given to the aperture is a stop taken from the shutter: same exposure, different photograph.
- Set the one the photograph is about, and let the other follow.