Exposing a scene

Correctly exposing a photograph is one of the most fundamental skills in film photography. The process involves balancing three key settings—ISO, shutter speed, and aperture—to achieve a well-lit image. Together, these settings form what is known as the exposure triangle.

Understanding exposure is essential for creative control, whether you're aiming for a crisp, well-detailed image or an intentionally moody, underexposed shot. This guide covers the core principles of exposure, metering methods, and even how to estimate exposure without a meter.

The exposure triangle

The exposure triangle consists of three interdependent settings that determine the final brightness of an image:

1. ISO – The film's sensitivity to light.
2. Shutter speed – How long the film is exposed to light.
3. Aperture – The size of the lens opening that controls light intake.

Balancing these three elements correctly ensures a properly exposed image. If you adjust one, you’ll need to change the others to keep the exposure correct.

ISO

ISO represents the film's sensitivity to light, traditionally known as film speed. It is measured using an ISO (or ASA) rating (e.g., 100, 400, 1600).

• Lower ISO (e.g., 50–200): Less sensitive to light, requiring more exposure. This produces finer grain and higher image quality.
• Higher ISO (e.g., 800–3200): More sensitive to light, requiring less exposure. Increases film grain but allows shooting in low-light conditions.

Film sensitivity is determined by the size of the silver halide grains in the emulsion. Higher ISO films have larger grains, making them more reactive to light but also producing more visible grain in the image.

Tip: While film ISO is fixed once loaded, it is possible to alter exposure of the entire roll by pushing or pulling during development.

How to set ISO on your camera

Since ISO cannot be changed between shots on film cameras, it must be set correctly when loading the film.

• Manual ISO dial (most SLRs & rangefinders)

  • Locate the ISO (or ASA) dial, usually near the rewind crank or shutter speed dial.
  • Lift and rotate the dial to match the ISO of your film.

• DX-coded cameras (most point & shoots & some SLRs)

  • These cameras automatically detect the film’s ISO from DX-coded canisters.
  • If using non-DX film, check if your camera allows manual ISO override.

• Fully manual cameras (older & mechanical models)

  • Some older cameras have no ISO setting. Instead, you must manually adjust shutter speed and aperture based on the film's rated ISO.

Setting the correct ISO ensures the camera’s light meter provides accurate exposure recommendations.

Shutter speed

Shutter speed controls the amount of time the film is exposed to light.

• Faster shutter speed (e.g., 1/1000s, 1/500s, 1/250s): Reduces motion blur, ideal for capturing fast-moving subjects.
• Slower shutter speed (e.g., 1/30s, 1s): Increases motion blur, useful for low-light shots and creative effects like light trails.
• Bulb mode (B) / Time mode (T): Keeps the shutter open for as long as the shutter button is pressed. Used for long exposures, such as night photography, star trails, or light painting.

Inside the camera, the shutter curtain (or leaf shutter in some cameras) physically opens and closes. A fast shutter speed means the curtain opens and shuts quickly, limiting light exposure. A slow shutter speed means the curtain stays open longer, increasing exposure.

Tip

Handheld shots at slower speeds (below 1/60s) risk camera shake, so use a tripod when necessary.

How to set shutter speed on your camera

Shutter speed is usually controlled by a dedicated dial or an electronic setting. On most cameras, the shutter speed dial only shows the denominator of the fraction. For example, "500" on the dial represents 1/500th of a second, and "60" represents 1/60th of a second.

• SLRs & rangefinders with a shutter speed dial

  • Many mechanical and electronic SLRs and rangefinders have a physical shutter speed dial.
  • Turn the dial to select the desired shutter speed, usually in full stops (e.g., 1/125s, 1/250s).

• Fully automatic & aperture priority cameras

  • In Aperture priority (A or Av) mode, the camera selects the shutter speed automatically based on your chosen aperture.
  • In Fully automatic (Auto) mode, the camera controls both aperture and shutter speed to achieve a balanced exposure.
  • Many point-and-shoot cameras only offer automatic exposure, meaning shutter speed cannot be manually adjusted.

Shutter speed should be set based on lighting conditions and subject movement.

Aperture

Aperture controls the size of the lens opening, determining how much light reaches the film. It is measured in f-stops (e.g., f/2.8, f/8, f/16). A lower f-number means a wider aperture, while a higher f-number means a smaller aperture.

Inside the lens, the diaphragm expands or contracts to adjust the size of the opening. A wide aperture creates a shallow depth of field, isolating the subject, while a small aperture increases depth of field, keeping more of the scene in focus.

Aperture-opening diagram

• Wide aperture (e.g., f/1.8, f/2.8): Allows more light in, creating a shallow depth of field (blurry background).
• Narrow aperture (e.g., f/11, f/16): Allows less light in, increasing depth of field (sharp background).

TIP

While stopping down increases depth of field, excessively small apertures (e.g., f/22, f/32) can cause diffraction, reducing overall image sharpness.

How to set aperture on your camera

Aperture is usually controlled by the lens or an electronic setting.

• SLRs & rangefinders with manual aperture control

  • Rotate the aperture ring on the lens to select the desired f-stop.

• Electronic cameras (some SLRs & point & shoots)

  • If the camera has a mode dial, select Aperture Priority (A or Av) and adjust aperture manually and have the camera set the shutter speed.
  • In Manual (M) mode, both aperture and shutter speed must be set manually.

• Fixed-lens cameras (point & shoots & some rangefinders)

  • Many compact cameras have a fixed aperture or limited manual control.

Aperture should be set based on lighting, subject isolation, and desired depth of field.

Metering in camera

Metering in photography refers to the process of measuring the light in a scene to determine the correct exposure settings—shutter speed, aperture, and ISO. This measurement is taken using a light meter, which evaluates the brightness of the subject and helps the photographer achieve a well-exposed image.

Most modern cameras have a built-in light meter, while older film cameras may require an external light meter or manual calculations to determine exposure.

Metering modes

Most cameras include built-in metering modes, which determine how light is measured and which settings are controlled automatically. The four main modes are:

1. Fully automatic (program mode) – The camera selects both shutter speed and aperture for a balanced exposure.
2. Shutter priority (S or Tv mode) – You set the shutter speed; the camera adjusts the aperture.
3. Aperture priority (A or Av mode) – You set the aperture; the camera adjusts the shutter speed.
4. Manual mode (M mode) – You control both settings, relying on a light meter for guidance.

Each mode offers varying levels of creative control. Manual mode is ideal for learning exposure fundamentals, while shutter or aperture priority can help when experimenting.

Meters inside a camera

Most analogue cameras built after the late 60s feature built-in light meters, which display exposure information through different mechanisms:

• Needle and dial meters – A mechanical needle moves up or down in response to light levels. The goal is to adjust settings until the needle aligns with a central mark.
• LED or LCD meters – Some electronic cameras use a series of LEDs or a numerical display to indicate whether the image is underexposed, correctly exposed, or overexposed.
• Fully automatic – Fully automatic cameras may not provide any indication of the shutter speed or aperture settings to be used, but may warn the user when a correct exposure cannot be achieved.

External meters

External light meters can offer greater precision and more consistency than built-in meters and come in two main forms:

1. Professional light meters

   • Used in film and studio photography.
   • Measures incident light (falling on the subject) rather than reflected light.
   • Provides accurate exposure values for complex lighting situations.

2. Smartphone light meter apps

   • Light meter apps use the phone’s camera to estimate exposure.
   • Less precise but convenient for quick readings.
   • Can typically be controlled a little more than built-in meters.

Metering by eye

Experienced photographers can estimate exposure without a light meter by using the simple and effective Sunny 16 Rule for daylight exposure:

• Set aperture to f/16 in bright sunlight.
• Set shutter speed to match ISO (e.g., ISO 100 → 1/100s, ISO 200 → 1/200s).

Adjust for different lighting conditions:

Condition	Aperture	100 ISO	200 ISO
Bright sunlight	f/16	1/100s	1/200s
Slightly overcast	f/11	1/100s	1/200s
Overcast	f/8	1/100s	1/200s
Heavy overcast	f/5.6	1/100s	1/200s
Shade	f/4	1/100s	1/200s

This method provides a solid starting point for correct exposure when a meter is unavailable.