All three together

Learning goal

You will be able to use the aperture, shutter speed and ISO together as a single decision. You will show by counting steps that the same brightness can be reached with different combinations, work out which setting you need to change, and by how much, to keep the brightness when you change one setting, weigh each setting's cost beyond light, that is sharpness, movement and grain, decide what you want to protect first in a scene and set the settings according to that priority, and choose deliberately which cost to pay in dim scenes with people of different skin tones.

The question

Imagine two photographers photographing a family under the same tree in the same park at the same minute. Both photos are exactly the same brightness. But in one, only the mother's face is sharp, and the children behind are a soft patch of color; in the other, the whole family is sharp, but the ball thrown in the air is a blurred line. The same light, the same brightness, two different photos.

How do the three settings affect each other? You can fill a bucket to the same level by opening the tap wider or by waiting longer; raising the reading needle can also bring the result to the same brightness. There are many paths to the same result, and each path leaves a different mark on the photo. In this lesson you will learn to read these paths and choose between them.

Context

In the previous three lessons you got to know the corners of the exposure triangle one by one. Besides light, the aperture decided the slice of sharpness, the shutter speed movement, and ISO, besides the signal, grain. In each of their series you saw that one step doubles or halves the brightness. This lesson puts the three series side by side and shows how trade-offs are made between them.

Photographers compare this trade-off to a budget. The light the scene offers you is a light budget; you share this budget between the aperture, shutter speed and ISO. When you give one extra light, you have to take it from another. We will see the common name photographers use for these steps, the "stop", in detail in the next lesson; for now we will count moving one rung forward or back in each setting's series as "one step".

Simple explanation

How do you reach the same brightness by different paths?

Imagine that in a scene the setting the camera suggests is f/5.6, 1/125 and ISO 200. If you open the aperture one step to f/4, you get twice the light; to keep the brightness you need to shorten the shutter speed one step to 1/250. One more step: f/2.8 and 1/500. In the other direction, if you stop the aperture down to f/8, the light halves; you restore the balance by lengthening the shutter speed to 1/60. All four combinations have the same brightness.

ISO joins this trade-off too. If you want to stop down to f/8 and keep the shutter speed at 1/125, you win back the missing step by raising ISO from 200 to 400. If you stop down to f/11, two steps go missing; ISO 800 makes up for these two steps. The calculation fits in a single sentence: however many steps of light you lose in one setting, you must gain the same number of steps in the others. This balance is called equivalent exposure. The camera's light meter suggests a single solution; but that solution's equivalents give the same brightness too, and the choice is in your hands.

What does each path cost?

Equivalent combinations give the same brightness but not the same photo. Opening the aperture gains light but narrows the slice of sharpness; the back row of a group can blur. Stopping the aperture down widens sharpness but loses light, and at very narrow openings softens sharpness through diffraction. Lengthening the shutter speed gains light but records movement and hand shake. Shortening the shutter speed freezes movement but loses light. Raising ISO brightens the result but makes grain visible.

So every step has a cost, a side effect, and the three settings' costs are paid in different currencies: sharpness, movement and cleanness. On a bright day the budget is plentiful; you can use all three settings comfortably as you like. On a dim evening, though, the budget is tight; you have to pay from somewhere. The real question is which cost you want to pay. Settings made without answering this question are like the camera paying from a random place: sometimes exactly the photo you wanted comes out, and sometimes a cost you didn't want at all has been paid.

What should you protect first?

The easiest way to decide is to ask what you want to protect first in the scene. In a portrait, what matters is often the slice of sharpness: if you want the face sharp and the background soft, you choose the aperture first, then keep the shutter speed short enough to freeze movement, and close the remaining gap with ISO. With a running child or at a match, what matters is movement: you choose the shutter speed first, then set the aperture and ISO to suit it. The setting you choose first is your priority setting.

In a landscape on a tripod, movement doesn't matter, but cleanness and deep focus do: you set ISO to its lowest, the aperture to a medium-narrow value around f/8, and lengthen the shutter speed as long as needed. This way of thinking about priority is also the basis of the A and S modes you will see at the end of this module: in A mode you choose the aperture, in S mode the shutter speed; the camera completes the rest according to this priority. A photographer who knows the priority also knows what to leave to the camera.

What cost is paid in dim light?

When people of different skins are in the same frame in a dim setting, the budget is both tight and the two faces' needs are different. A dark face sends back less light than the light face next to it; to keep the detail of both together, the exposure usually needs to be set a little more generously for the dark face. This extra light has to be taken from somewhere. Lengthening the shutter speed can blur the faces; opening the aperture a lot can push one of the two people outside the slice of sharpness.

In a scene like this, the best trade-off is often this: an aperture enough to keep both faces inside the slice of sharpness, a shutter speed short enough to freeze the faces, and a higher ISO for the missing light. A little grain is less of a loss than a blurred face or skin buried in darkness. Sometimes the best solution isn't in the settings but in the light: turning the two people a little toward the window enlarges the budget, and you pay less in every setting.

Visual evidence

On an autumn afternoon, you photograph a family having a picnic under a big plane tree in the park. The mother is in front, two children playing with a ball two steps behind her, and the father behind them. The light comes softly through the trees. First you shoot at f/2, 1/1000 and ISO 100. The mother's face is sharp, and her light brown skin and the lines of her smile can be made out; the children and the father behind have turned into warm patches of color. The ball frozen in the air is a patch too; it has stayed outside the slice of sharpness.

Then you stop the aperture down four steps to f/8 and, to keep the brightness, lengthen the shutter speed four steps to 1/60. Now the whole family is sharp: the children's faces, the father's dark skin and beard can be made out. But the ball in the air is a blurred line at 1/60; the running child's arms have softened slightly too. Finally, staying at f/8, you shorten the shutter speed three steps to 1/500 and win back the three missing steps by raising ISO to 800. The whole family is sharp, the ball frozen in the air; but looking closely, there is a fine grain over the photo. Three frames, the same brightness; a different cost has been paid in each.

Common mistakes

Summary

The aperture, shutter speed and ISO share the same light budget: if every step lost in one is won back with the same number of steps in the others, the brightness doesn't change, and one of these equivalent combinations can be chosen. But each combination costs something different: the aperture is paid in sharpness, the shutter speed in movement, ISO in grain. Deciding before shooting what you want to protect sets the order of the settings; in dim light with people of different skins, sharing the cost between the three settings or changing the light keeps every face sharp and in its own color.

Mini assignment

In this assignment you will photograph the same scene with three equivalent combinations; you need a manual shot where you set all three settings.

1. Set up a scene with something near, something far and a moving element: a cup in front, a bookcase behind, and water being poured into the cup.
2. Note the setting the camera suggests, then work out three combinations that give the same brightness: one with a wide aperture, one with a narrow aperture and a long shutter speed, one with a narrow aperture and a high ISO.
3. Take the three frames and check whether the brightness stayed the same.
4. Under each frame, write which cost you paid: sharpness, movement or cleanness.

Check yourself

• Is the brightness of the three frames really the same, or did you count the steps wrong?
• Can you name in one word the cost paid in each frame?
• If you were photographing a person, which combination would you choose to keep the face sharp and in its own color?

Glossary

Mini quiz

1. Which combination gives the same brightness as f/5.6, 1/125, ISO 200?

• A) f/4, 1/125, ISO 200
• B) f/4, 1/250, ISO 200
• C) f/8, 1/250, ISO 200
• D) f/5.6, 1/250, ISO 100

2. If you want to stop the aperture down two steps and keep the shutter speed the same, how should you change ISO?

• A) Raise it two steps
• B) Lower it two steps
• C) Not change it
• D) Lower it one step

3. When photographing a running child, which setting is chosen first?

• A) ISO
• B) White balance
• C) Shutter speed
• D) Aperture

4. When photographing two people, one dark-skinned and one light-skinned, in dim light, which trade-off often gives the best result?

• A) Closing the whole gap by lengthening the shutter speed
• B) Opening the aperture to its widest and leaving one person outside the sharpness
• C) Leaving the dark-skinned person in darkness
• D) An aperture that keeps both faces sharp, a time that freezes the faces, and a higher ISO for the missing light

Answers

1. B — When the aperture opens one step, twice the light comes in; shortening the shutter speed one step takes this extra back and the brightness stays the same.
2. A — The aperture loses two steps of light; for the same brightness you win these two steps back by raising ISO two steps.
3. C — With a running child, what needs protecting is movement; the time that will freeze it is chosen first, and the other settings are set to suit it.
4. D — Sharing the cost between the three settings keeps both faces sharp and visible; a little grain is less of a loss than a lost face.

Summary card

• The same brightness can be reached with many combinations: equivalent exposure.
• Gain in the others the same number of steps you lost in one.
• The costs differ: the aperture is paid in sharpness, the shutter speed in movement, ISO in grain.
• Decide first what you will protect; choose that setting first.
• In dim light, share the cost or change the light; let every face stay sharp and in its own color.

Bridge to the next lesson

In this lesson we counted light in "steps": one step twice, one step half. Photographers give these steps a common name, and when one photographer tells another to "open up a stop", the same amount of light is meant whichever setting they touch. The next lesson explains this common unit, the stop, and how it makes measuring light easier.