Exposing a scene

Exposure is about controlling how much light reaches your film. You balance three settings to get a well-lit image: ISO, shutter speed, and aperture. This guide covers the exposure triangle, how cameras measure light, and a backup method when meters fail.

The exposure triangle

Three settings work together to control brightness:

1. ISO: how sensitive the film is to light
2. Shutter speed: how long the film is exposed
3. Aperture: the size of the lens opening

Changing one affects the others. Get all three balanced and you get a correctly exposed image.

Shutter speed, aperture and ISO traded against each other, and what they add up to

Each step outward on an axis is one stop, and a stop doubles the light.

Watch the bar underneath. Trade a stop of shutter speed for a stop of aperture and it stays where it is, because the exposure has not changed. Change one setting on its own and the bar runs past the mark, or falls short of it.

ISO

ISO is the film's sensitivity to light, printed on the box. A higher number needs less light.

Once the film is loaded, its ISO is fixed for the whole roll. You can deliberately rate the roll at a different ISO and adjust the development to match. This is called push or pull processing, and it has to be decided before you shoot, not after.

Setting the ISO

• Manual dial (SLRs & rangefinders): find the ISO dial near the rewind crank or shutter speed dial. Lift it and rotate to match your film
• DX-coded (most point & shoots): the camera reads the film canister automatically. No setting needed
• Fully manual (older cameras): no ISO dial. You'll adjust shutter speed and aperture based on your film's rated ISO

Shutter speed

Shutter speed controls how long the film is exposed.

Many film cameras use a focal-plane shutter, made of two curtains that travel across the film gate. The first curtain opens to begin the exposure, and the second curtain follows to end it. Winding on resets both curtains, ready for the next frame.

Curtain 1 opens, curtain 2 closes, then both rewind

At fast shutter speeds the second curtain sets off before the first has finished crossing the gate. Only a narrow slit travels across the frame, so the film is never fully uncovered at once.

At fast speeds a thin slit sweeps across the frame

Speed	Effect	Use for
Fast (1/500s, 1/1000s)	Freezes motion	Sports, wildlife
Medium (1/125s, 1/250s)	Freezes some motion	General photography, slow or stationary subjects
Slow (1/30s, 1s)	Adds blur	Light trails, creative effects
Bulb (B)	Stays open while you hold it	Long exposures, night photography

Handheld shots blur below 1/30s

Camera shake softens the frame and you cannot fix it later. Use a tripod, or brace yourself against something solid.

The dial usually shows only the denominator, so "500" means 1/500th of a second.

Setting the shutter speed

• Manual dial: turn to select your speed (for example 1/125s or 1/250s)
• Auto mode: the camera picks for you
• Aperture priority (A/Av): you set aperture; camera sets shutter speed

Aperture

Aperture controls the size of the lens opening. That one setting decides two things: how much light reaches the film each second, and how much of the scene comes out sharp.

Inside the lens sits a ring of overlapping blades called the diaphragm. They swing together to change the size of the hole the light passes through. The setting is measured in f-stops, written f/2.8 or f/16. The number is a ratio rather than a width, so a bigger number means a smaller hole.

An aperture opening from a small f-stop to wide open and back

A wider hole lets more light through for every second the shutter stays open. Each full f-stop along the scale doubles or halves that light. That is the same size of step as one setting on the shutter speed dial. Open up one stop and you can shorten the shutter by one, and the exposure lands in the same place.

The same opening sets the depth of field. That is the band of distance either side of the focus point that comes out sharp. A wide opening makes that band shallow, so a subject stands out against a soft background. A narrow one makes it deep, and a scene can be sharp from a few metres away to the horizon.

The same aperture, and what it does behind the subject

Depth of field is not a fixed amount for a given f-stop. It also depends on how far away you focus, and it grows quickly with that distance.

Focus a 50mm lens at f/2 on a face a metre away and the sharp band is about four centimetres deep. Focus the same lens, at the same aperture, on a group ten metres away and the band is about four metres deep. Double the distance to your subject and the sharp band roughly quadruples.

So aperture is a strong control over a close subject, and a weak one over a distant scene. The depth of field calculator works this out for any lens, aperture and distance.

Aperture	Light	Depth of field	Use for
f/1.4 to f/2.8	Most	Shallow	Portraits, low light
f/4 to f/8	Middle	Moderate	General photography
f/11 to f/22	Least	Deep	Landscapes, bright light

INFO

Small apertures such as f/22 and f/32 can cause diffraction, which softens the whole frame.

Setting the aperture

• Manual lens ring: rotate to your desired f-stop
• Auto or priority modes: the camera may adjust automatically, or you set it and let the camera handle the rest

Metering in camera

Your camera measures light to suggest exposure settings. Most built after the 1960s have a light meter inside.

Exposure modes

Mode	What you set	What camera sets
Program (P)	Nothing	Aperture + shutter
Shutter priority (S/Tv)	Shutter speed	Aperture
Aperture priority (A/Av)	Aperture	Shutter speed
Manual (M)	Both	Nothing

For learning, start with Manual or Aperture priority so you understand how each setting affects the image.

Reading the meter

Different cameras display readings differently:

• Needle meters: change the shutter speed or aperture until the needle sits on the centre mark
• LED/LCD meters: look for a + or - indicator; adjust until exposure is balanced
• Fully automatic: fires anyway, or shows a warning if it cannot find a good exposure

When the meter gets it wrong

A meter assumes every scene is a middle grey. Snow, beaches and bright skies trick it into giving too little light. A dark room or a black cat tricks it into giving too much.

With negative film, when in doubt give one stop more light. Negative film handles too much light far better than too little.

External meters

For more accuracy, especially in tricky lighting:

• Professional meters: measure incident light (light falling on your subject) for precision
• Smartphone apps: less accurate but convenient and more adjustable than built-in meters

Metering by eye

When you have no meter, use the sunny 16 rule:

1. Set aperture to f/16 on sunny days
2. Set the shutter speed nearest your film's ISO: ISO 100 becomes 1/125s, and ISO 400 becomes 1/500s

Condition	Aperture
Bright sun	f/16
Slight overcast	f/11
Overcast	f/8
Heavy overcast	f/5.6
Deep shade	f/4

Use this as a starting point. Check your results and adjust from there.