Photography Fundamentals — The Exposure Triangle: Shutter Speed, Aperture, ISO

What is exposure?

Exposure is what determines how bright a photo looks. A photo that’s too bright is overexposed; one that’s too dark is underexposed.

The three factors that combine to produce correct exposure are known as the exposure triangle.

     Exposure Triangle
           ISO
          /    \
  Shutter Speed ─── Aperture

These three elements have to balance against each other to produce a photo with the brightness you want.

Shutter speed

Shutter speed is the length of time the shutter stays open, which determines how long light reaches the sensor.

How it’s notated

1/4000s → 1/2000s → 1/1000s → 1/500s → 1/250s → 1/125s
Fast ←←←←←←←←←←←←←←←←←←←←←←←←←←←→→→→ Slow
Darker ←←←←←←←←←←←←←←←←←←←←←←←←←←←→→→ Brighter

Shutter speed’s two roles

1. Controlling brightness: the slower it is, the more light comes in → the brighter the photo

2. Rendering motion:

Shutter speed	Effect	Good for
1/1000s or faster	Freezes the instant (subject stays sharp)	Sports, birds, water droplets
1/250s	Freezes typical motion	Someone walking, a moving car
1/60s	Slight blur possible	Portraits (the handheld limit)
1/30s or slower	Motion blur	Waterfalls, light trails
Several seconds or more	Full light trails	Star trails, night scenes

Avoiding handheld blur: as a rule of thumb, use at least 1/(focal length)s. With a 50mm lens, that means at least 1/50s.

Aperture

The size of the light-passing opening inside the lens is expressed as an f-number (f-stop).

The counterintuitive relationship of f-numbers

f/1.4  f/2  f/2.8  f/4  f/5.6  f/8  f/11  f/16
←── Larger opening ───────────────────────── Smaller opening ──→
←── Brighter ─────────────────────────────────── Darker ──→

Note: the smaller the f-number, the larger the opening — it’s an inverse relationship.

Depth of field

Aperture’s second role is determining the range that stays in focus (depth of field).

f-number	Depth of field	Background	Use
f/1.4–f/2.8	Shallow	Strong blur (bokeh)	Portraits, product photography
f/4–f/5.6	Moderate	Moderately blurred	Everyday snapshots
f/8–f/11	Deep	Sharp throughout	Landscapes, architecture
f/16 and beyond	Very deep	Very sharp	Starburst effects

ISO (sensitivity)

This is how sensitive the camera’s sensor is to light.

ISO 100 → ISO 200 → ISO 400 → ISO 800 → ISO 1600 → ISO 3200 → ISO 6400
Low sensitivity                                                High sensitivity
Darker                                                          Brighter
Less noise                                                      More noise

Choosing an ISO

Situation	Recommended ISO
Bright outdoors	ISO 100–200
Overcast/shade	ISO 400
Indoors (bright lighting)	ISO 800–1600
Indoors (dim lighting)	ISO 3200–6400
Night/concerts	ISO 6400 or higher

Rule of thumb: keep ISO as low as possible, and handle exposure first through shutter speed and aperture.

How the exposure triangle interacts

The three elements are all linked together. Change one, and you need to adjust the other two to maintain the same brightness.

The concept of a stop

One stop means doubling or halving the amount of light.

Shutter: 1/500 → 1/250 (1 stop slower) → 2x the light
Aperture: f/5.6 → f/4 (1 stop wider) → 2x the light
ISO: ISO 200 → ISO 400 (1 stop up) → 2x the sensitivity

Sample optimal settings by situation

Portraits (outdoors, sunny)

Shutter: 1/250s (freezes natural movement)
Aperture: f/2.8 (blurs the background)
ISO: 100–200 (minimizes noise)

Landscapes (using a tripod)

Shutter: 1/30s–several seconds (tripod required)
Aperture: f/8–f/11 (overall sharpness)
ISO: 100 (lowest noise)

Sports/wildlife photography

Shutter: 1/1000s or faster (freezes motion)
Aperture: f/4–f/5.6 (moderate background separation)
ISO: 800–3200, depending on the situation

Night cityscapes (tripod)

Shutter: 5–30 seconds (captures light trails)
Aperture: f/8 (overall sharpness plus starburst)
ISO: 100–400 (minimizes noise)

Reading the histogram

A histogram is a graph showing the brightness distribution in a photo.

▓▓▓
▓▓▓▓▓▓▓▓
▓▓▓▓▓▓▓▓▓▓▓▓▓
▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓
Dark Mid Bright
0 255

Histogram shape	Meaning
Skewed left	Underexposed (dark photo)
Skewed right	Overexposed (bright photo)
Evenly spread	Proper exposure
Clipped at right edge	Highlight clipping (unrecoverable)
Clipped at left edge	Shadow clipping

Goal: keep the histogram from clipping at either end.

Practice quiz — 5 questions

Q1. What settings are needed to render a moving object sharply?

① Slow shutter speed + low ISO
② Fast shutter speed + high ISO
③ Low f-number + slow shutter speed
④ High f-number + fast shutter speed

Answer: ② (a fast shutter freezes the motion; the resulting darkness is compensated with ISO)

Q2. Between f/1.8 and f/11, which produces a more blurred background?

① f/1.8 ② f/11 ③ Both the same ④ Can’t tell

Answer: ① (a lower f-number means a shallower depth of field → more background blur)

Q3. What side effect appears when you raise ISO?

① The photo gets darker
② The colors fade
③ Noise (graininess) increases
④ The focus goes soft

Answer: ③

Q4. Which of the following correctly describes one stop?

① A 10x increase in shutter speed
② A 2x or 1/2x change in the amount of light
③ A 100-point increase in ISO
④ A 0.5 increase in aperture

Answer: ②

Q5. What does it mean when a histogram is clipped at the right edge?

① The photo is too dark
② There’s a lot of noise
③ The highlights are blown out and unrecoverable
④ The shutter speed was too fast