Photography 101

Lesson 1: exposure

The exposure
triangle

Change aperture, shutter speed and ISO the way you would on a camera, one third of a stop per click. The viewfinder shows how bright the picture gets, and what each setting does to it.

f/41/250ISO100
Background (aperture)
Bird (shutter)
Noise (ISO)

Simplified. Real background blur also depends on the lens focal length and how far away the subject is. Real motion blur depends on how fast the subject moves.

0 stops
Same brightness as the baseline

Baseline exposure: f/4, 1/250 s, ISO 100.

f/4
0
f/1, more lightf/16, less light
1/250
0
1/8 s, more light1/4000 s, less light
100
0
100, cleanest6400, brightest

Load an example

Trade one stop for another

  1. Start at f/4, 1/250 s, ISO 100.
  2. Turn aperture three clicks: f/4, f/4.5, f/5, f/5.6. You have lost exactly one stop, and the picture gets darker.
  3. Turn shutter speed three clicks the other way: 1/250, 1/200, 1/160, 1/125. You gain one stop back.
  4. The meter is back at 0. Brightness is the same, but the background is sharper and the bird is more blurred.

What a stop is

One stop means twice the light, or half of it. Most cameras move in thirds of a stop by default (some let you switch to half stops), so three clicks of a dial make one full stop. Each click changes the light by 21/3 ≈ 1.26×, and three clicks give 1.26³ ≈ 2×.

ISO works differently. It doesn't change how much light reaches the sensor. It amplifies the signal, so one ISO stop doubles the brightness of the picture, not the light.

Aperture numbers follow their own pattern. One click multiplies the f-number by about 1.12, and the sections below show why.

One stop brighterBaselineOne stop darker
Aperturef/2.8f/3.2, f/3.5 betweenf/4f/5.6
Shutter1/1251/160, 1/200 between1/2501/500
ISO200125, 160 between10050only on some cameras

Why f-stops have strange numbers

The standard full-stop aperture sequence is f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Why isn't it simply 1, 2, 3, 4? Each circle below is drawn to scale for the same lens, and each one lets in half the light of the one before it.

Light follows the area

The f-number is the lens focal length divided by the diameter of the opening. On a 50 mm lens, f/2 is a 25 mm opening and f/4 is 12.5 mm. That is why a bigger number means a smaller opening.

The amount of light depends on the area of the circular aperture:

A = πr2

To halve the area, the diameter doesn't need to halve. It only needs to shrink by a factor of

√2 ≈ 1.414

That's where the f-stop sequence comes from.

Build the sequence yourself

Take f/2 and multiply by 1.414:

2 × 1.414 = 2.828 ≈ 2.8

So the next full stop is f/2.8. Then:

2.8 × 1.414 ≈ 4

Keep going and you get f/2, f/2.8, f/4, f/5.6, f/8. Each step gives half as much light.

It feels backward at first

f/2 to f/2.8
−1 stop of light
f/2.8 to f/2
+1 stop of light

A smaller f-number means a larger physical aperture, and more light.

Modern cameras click in thirds

Modern cameras usually adjust aperture, shutter speed and ISO in 1/3-stop increments, not only full stops. Each click is 1/3 stop, so three clicks make one full stop. Your camera may show:

Aperture

  1. f/2.8
  2. f/3.2
  3. f/3.5
  4. f/4

3 clicks, 1 stop less light

Shutter speed

  1. 1/125
  2. 1/160
  3. 1/200
  4. 1/250

3 clicks, 1 stop less light

ISO

  1. 100
  2. 125
  3. 160
  4. 200

3 clicks, 1 stop brighter

One click of light

A full stop doubles or halves the light. A third of a stop changes it by

21/3 ≈ 1.26

So each 1/3-stop click is roughly 1.26× the light, rather than 2×. Three clicks give 1.26 × 1.26 × 1.26 ≈ 2.

One click of aperture

Light goes with the diameter squared, so the f-number only needs to change by the square root of one click of light, √(21/3):

fnext = fcurrent × 21/6

That multiplier is about 1.122. Starting from f/2.8:

  1. 2.8 × 1.122 ≈ 3.14, marked f/3.2
  2. 3.2 × 1.122 ≈ 3.59, marked f/3.5
  3. 3.5 × 1.122 ≈ 3.93, marked f/4

Why is 3.14 marked f/3.2 and not f/3.1? The true value of f/2.8 is 2.83 (2 × √2), and 2.83 × 1.122 ≈ 3.17. The same happens with f/3.5, whose true value is 3.56, so the next click lands exactly on 4.

The 1/3-stop aperture sequence

The numbers on screen are rounded labels, and the camera uses the exact values. For example f/5.6 is really 5.66, f/11 is 11.3, and 1/125 s is really 1/128 s. The practical sequence is:

  1. f/1.4
  2. f/1.6
  3. f/1.8
  4. f/2
  5. f/2.2
  6. f/2.5
  7. f/2.8
  8. f/3.2
  9. f/3.5
  10. f/4
  11. f/4.5
  12. f/5
  13. f/5.6
  14. f/6.3
  15. f/7.1
  16. f/8
  17. f/9
  18. f/10
  19. f/11
  20. f/13
  21. f/14
  22. f/16

Bold values are full stops.