Three Dials, One Budget

A camera has an alarming number of buttons, and almost all of them are conveniences. The actual picture is decided by three settings: aperture, shutter speed, and ISO. Everything else is either automating those three or adjusting how the file is processed afterwards.

The useful way to think about them is as a budget. The sensor needs a certain amount of light to produce a correctly exposed image. Each of the three settings can supply more or less of it — and each one changes something else about the picture as a side effect. Photography, in practical terms, is choosing which side effects you want.

Aperture — the Hole, and the Depth of Field

The aperture is an adjustable opening inside the lens. Wider opening, more light. It’s measured in f-stops: f/1.8, f/2.8, f/4, f/5.6, f/8, f/11, f/16.

The first thing that trips everyone up is that the numbers run backwards. f/1.8 is a wide opening letting in a lot of light. f/16 is a narrow one letting in very little. The reason is that the number is a ratio — focal length divided by the diameter of the opening — so a bigger denominator means a smaller hole. You don’t have to internalise the maths; you have to internalise that small number equals lots of light.

The side effect is depth of field: how much of the scene, front to back, appears acceptably sharp.

  • Wide aperture (f/1.8 – f/2.8) — shallow depth of field. A portrait subject is sharp, the background dissolves into soft blur. This is the look most people buy a “proper camera” hoping to get, and it’s mostly an aperture decision, not a camera one.
  • Middle (f/5.6 – f/8) — a comfortable general-purpose range. Enough depth to keep a group of people or a whole subject sharp. Most lenses are also at their optically sharpest somewhere around here.
  • Narrow (f/11 – f/16) — deep depth of field. Foreground rocks and distant mountains both sharp, which is why landscape photographers live here.

Worth knowing: past about f/16 on most cameras, images start getting softer again due to diffraction. Stopping down further is not a free upgrade in sharpness.

Shutter Speed — the Duration, and the Motion

Shutter speed is how long the sensor is exposed, in seconds or fractions of a second: 1/1000, 1/250, 1/60, 1/15, 1", 5".

Longer exposure, more light — and more time for anything to move during it.

  • 1/1000s and faster — freezes fast motion. Sports, birds in flight, splashing water.
  • 1/250s — freezes people walking, most casual movement.
  • 1/60s — around the limit of hand-holding for many people before camera shake shows up, and slow enough that a moving hand or a running child blurs.
  • 1/15s and slower — needs a tripod or stabilisation. This is where waterfalls turn silky and car headlights become streaks.

The old rule of thumb for hand-holding is 1/focal length: with a 50mm lens, don’t go below about 1/50s; with a 200mm lens, stay above 1/200s. Modern image stabilisation buys you several stops beyond that, but stabilisation only counters your shake — it does nothing about a subject who is moving.

ISO — the Amplifier, and the Noise

ISO controls how strongly the sensor’s signal is amplified. Higher ISO means a brighter image from the same amount of light, which sounds like a free lunch and isn’t.

The side effect is noise: a grainy, speckled texture, worst in shadows and in flat areas like skies, along with a loss of fine detail and some colour accuracy as the camera’s noise reduction works to hide it.

  • ISO 100–400 — clean. Use whenever there’s enough light.
  • ISO 800–3200 — visible noise on close inspection, entirely usable, and modern sensors handle this range far better than cameras from a decade ago.
  • ISO 6400 and up — noticeable degradation, but a noisy photograph of a moment beats a clean photograph of a blur. This is a rescue tool, not a failure.

The practical rule: keep ISO as low as the other two settings allow, and raise it without guilt when the alternative is a shot you can’t take at all.

How the Trade Works in Practice

Each of the three moves in stops. One stop means double or half the light. Going from f/8 to f/5.6 is one stop more light. From 1/250s to 1/125s is one stop more. From ISO 400 to ISO 800 is one stop more. Because they share a unit, you can trade freely — open the aperture one stop and you can shorten the shutter by one stop with an identical exposure but a shallower depth of field.

That’s the whole system. Everything else is deciding which side effect the picture needs.

Which Mode to Actually Use

Full manual is not the goal, and shooting in it doesn’t make anyone a better photographer. The semi-automatic modes exist because they’re genuinely better for most situations.

Aperture priority (A or Av) — you pick the aperture and ISO, the camera picks the shutter speed. This is what a large number of working photographers use most of the time, because depth of field is usually the creative decision and correct exposure is usually just arithmetic. Start here.

Shutter priority (S or Tv) — you pick the shutter speed, the camera handles the aperture. Useful when motion is the point: sports, kids, panning shots.

Manual (M) — you set all three. Worth it when the light isn’t changing and you want consistency across many frames — studio work, concerts, astrophotography — or when the camera’s meter is being fooled by a very bright or very dark scene.

Auto ISO with a minimum shutter speed is the setting that quietly solves most beginner problems. In aperture priority, set the aperture you want, tell the camera never to drop below 1/125s, and let ISO float. You get the depth of field you chose, no motion blur, and only as much noise as the scene forces.

A Practical Starting Set

Walking around in daylight: aperture priority, f/5.6, ISO 200, auto ISO with a 1/200s floor.

Indoors without flash: aperture priority, widest aperture your lens has, auto ISO with a 1/60s floor, and accept the noise.

Portraits outdoors in shade: aperture priority, f/2 to f/2.8, ISO as low as it will go.

Set those, take a hundred photographs, then start changing one setting deliberately and watching what happens. The trade only becomes intuitive by watching it play out in your own pictures.