A photograph is light, collected. You only have two ways to change how much of it arrives: make the hole in the lens bigger, or keep that hole open for longer. The camera calls them aperture and shutter speed. This is what they do, in plain words.
First, what a stop means
Photographers count light in stops. One stop up is twice as much light. One stop down is half. That is the whole idea.
Aperture moves in stops. Shutter speed moves in stops. ISO moves in stops. Because they all use the same unit, you can take light away from one and give it back with another. That is why a photographer can say “give me one more stop” without saying which dial to turn.
Two stops is four times, not twice
Stops stack up by doubling, so they run away from you fast. One stop is twice the light. Two stops is four times. Three is eight, and five is thirty two, the same way a folded sheet of paper doubles its own thickness with every fold.
Drag the slider and watch the squares fill up. This is why photographers talk in stops instead of percentages: two stops is a sentence, and four hundred percent is arithmetic.
2 stops up is 4× the light
Aperture: the size of the hole
Inside the lens there is a ring of thin blades that slide over each other. They leave a hole in the middle, and that hole is the aperture. Think of the pupil in your eye: wide in a dark room, small in bright sun. Same job.
The hole is measured with 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 row lets in half the light of the step before it.
Drag the dial and watch the blades move. The orange disc is the light getting through. It does not shrink gently. Every notch throws away half of what was there.
Why the numbers run backwards
Here is the bit everyone trips on once: small number means big hole. f/1.4 is wide open and lets in a lot of light. f/22 is nearly shut.
It reads backwards because the f-number is not a size, it is a sum. Take the length of the lens and divide it by the width of the hole. A 50mm lens with a 25mm hole is 50 divided by 25, which is f/2. Close that hole to 6mm and the same lens is at f/8. The hole got smaller, so the number got bigger.
Drag the slider in the drawing and watch the opening rather than the label. If it helps, read f/8 as “one eighth of the lens” and f/2 as “one half”. A half is bigger than an eighth.
50 ÷ 12.5 = f/4
Why the wide end is what you pay for
Not every lens can reach every f-number, and this is where the price of a lens shows up. Nearly all of them close down to f/16 or f/22. It is the other end, wide open, that costs money.
The zoom that came in the box with the camera usually opens to about f/3.5 at its widest setting, and only to f/5.6 once you zoom in. That is called a variable aperture, and it is printed on the barrel as something like 18-55mm f/3.5-5.6. Zoom in and the lens quietly costs you a stop and a half of light, right when you were probably struggling to hold the camera steady anyway.
Spend more and you get a zoom that holds f/2.8 the whole way through, or a lens that does not zoom at all and opens to f/1.8 or f/1.4. That is two to three stops more light than the kit zoom. It is the reason the expensive lens still works indoors, at dusk, and in a sports hall, where the cheap one gives up and asks for ISO.
You are paying for glass. A wider hole needs bigger pieces of it, ground more precisely, held in a heavier barrel. That is the honest reason an f/2.8 zoom costs what it costs and weighs what it weighs. The cheap way in is to give up zooming: a plain 50mm f/1.8 is one of the least expensive lenses made, and it opens wider than almost any zoom at any price.
What aperture costs: how much stays sharp
If aperture only changed brightness, you would leave it wide open and never think about it again. But it also decides how much of the scene is in focus. That is called depth of field, and it is usually the real reason to pick one f-number over another.
Wide open, only a thin slice of the scene is sharp. On an 85mm lens at f/1.8, focused on something two metres away, that slice is about four centimetres deep. That is not enough to keep both of a person’s eyes sharp if they turn their head. Photographers use it for portraits anyway, and on purpose: the face stands out and the wall behind it melts into soft colour.
Close the hole down to f/11 or f/16 and the sharp zone stretches from the grass at your feet to the hills in the distance. That is why landscapes live at that end of the scale. One more thing worth knowing: most lenses are at their very sharpest in the middle, around f/8.
Shutter speed: how long the camera looks
The shutter is a little curtain sitting in front of the sensor. Shutter speed is simply how long it gets out of the way: 1/1000 of a second, 1/60, or four whole seconds.
It counts in stops too, exactly like aperture. 1/500 is half the light of 1/250, which is half the light of 1/125. One notch, one stop. Notice that these are fractions as well, so the same rule applies: the bigger the bottom number, the shorter the look.
Press the button in the drawing. The ball crosses the frame the whole time. What the picture keeps is wherever the ball went while the curtain was open. Quick, and you get a ball. Slow, and you get a stripe, because the ball really was in all of those places while the camera was watching.
What shutter speed costs: movement
Some numbers worth remembering. About 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 starts to smudge.
Then there is your own wobble. The old rule: handheld, keep the shutter faster than one over the number on the lens. So 1/200 on a 200mm lens, and 1/50 on a 50mm. On a smaller-sensor camera, multiply the lens number by 1.5 first, so 200mm wants about 1/300. Stabilisation buys back a few stops, but it steadies your hands, not the thing you are photographing.
Blur is not always a mistake. 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 picture does. Long exposures turn a waterfall into silk and a crowd into fog. Slow is a fine choice, as long as you chose it.
The trade: one goes up, the other comes down
Here is the rule everything rests on. Open the aperture by one stop and halve the time the shutter stays open, and exactly the same amount of light lands on the sensor.
So f/8 at 1/125 gives the same brightness 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 reaching 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 one do you set first?
Whichever one the photograph is about. Let the camera work out the other.
If the picture is about movement, set the shutter. Pick the speed that freezes or smears the thing you care about, and let the aperture land wherever it lands. Your camera has a mode for exactly this, marked S or Tv on the dial.
If the picture is about what is sharp and what is soft, set the aperture. Pick how much you want in focus and let the shutter follow. That mode is marked A or Av, and it is where most photographers spend most of their time.
The camera is happy to choose both for you. It will pick the safe middle every time, and the safe middle is how you end up with a perfectly exposed photograph of nothing in particular.
The third dial
Everything above assumes one more setting is holding still: ISO, which is how sensitive the sensor is. When the light runs out and both your other controls are already at their limits, ISO makes the picture brighter and charges you in grain. It moves in stops like the other two, so it joins the same trade. The three together are called the exposure triangle, and they have their own piece.
The cheat sheet: one stop at a time
This is the one to keep. Every step down the table is one stop, which means twice as much light, on whichever dial you are turning. Count rows to count stops. To trade, move down a few rows in one column and up the same number in another: give the light away here, take it back there, and the picture stays the same brightness.
| Stops | Aperture | Shutter | ISO |
|---|---|---|---|
| 0 | f/45 | 1/8000 | 25 |
| 1 | f/32 | 1/4000 | 50 |
| 2 | f/22 | 1/2000 | 100 |
| 3 | f/16 | 1/1000 | 200 |
| 4 | f/11 | 1/500 | 400 |
| 5 | f/8 | 1/250 | 800 |
| 6 | f/5.6 | 1/125 | 1600 |
| 7 | f/4 | 1/60 | 3200 |
| 8 | f/2.8 | 1/30 | 6400 |
| 9 | f/2 | 1/15 | 12800 |
| 10 | f/1.4 | 1/8 | 25600 |
| 11 | f/1 | 1/4 | 51200 |
An example of reading across. You are at f/8, 1/250, ISO 800, and you want the background softer. Open the aperture two stops to f/4, then pay for it somewhere: two stops of shutter, from 1/250 to 1/1000, or two stops of ISO, from 800 down to 200. Same brightness, softer background. Aperture and shutter are free to move; ISO is the one you spend last, because it brings grain with it.
Where to start, by subject
There is no law here, and the light has not read any charts. But most subjects have a pair of settings photographers reach for first and adjust from there. Set the one in the last column 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.