Exposure Fundamentals

Every photograph is a record of light. Understanding how to control that light — how much reaches the film and for how long — is the foundation of photography.

This guide explains the exposure triangle: aperture, shutter speed, and ISO. Once you understand how these three elements interact, you can make intentional creative choices rather than guessing.

The Exposure Triangle

Three settings control exposure:

1. Aperture — How wide the lens opens (controls light volume)
2. Shutter Speed — How long light hits the film (controls light duration)
3. ISO — How sensitive the film is to light (fixed per roll)

These three work together. Change one, and you must adjust another to maintain the same exposure.

Aperture

Aperture is the opening in the lens that lets light through. It's measured in f-stops.

F-Stop Numbers

The confusing part: smaller numbers = larger openings.

Each full stop halves (or doubles) the light:

• f/2 lets in twice as much light as f/2.8
• f/8 lets in half as much light as f/5.6

Why the Odd Numbers?

F-stops are ratios of focal length to aperture diameter. The numbers represent the denominator of that fraction. f/2 means the aperture diameter is half the focal length.

The sequence (1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22) follows a pattern where each number is roughly the previous multiplied by √2 (about 1.4). This is because area (which determines light) scales with the square of diameter.

You don't need to remember the maths — just know the sequence.

Aperture and Depth of Field

Aperture controls more than exposure. It determines depth of field — how much of the scene is in focus.

Wide aperture (f/1.4 - f/2.8):

• Shallow depth of field
• Subject sharp, background blurred
• Classic portrait look
• Requires precise focusing

Medium aperture (f/5.6 - f/8):

• Moderate depth of field
• Good balance for most situations
• Where most lenses are sharpest

Small aperture (f/11 - f/22):

• Deep depth of field
• Near and far objects both sharp
• Landscape photography
• May show lens diffraction at smallest apertures

Shutter Speed

Shutter speed is how long the shutter stays open, exposing film to light. It's measured in fractions of a second.

Common Shutter Speeds

Each step doubles (or halves) the exposure time:

• 1/125 is twice as long as 1/250
• 1/60 is half as long as 1/30

Shutter Speed and Motion

Fast shutter speeds freeze motion. Slow shutter speeds blur motion.

Freezing action:

• 1/500+ for running, cycling
• 1/1000+ for birds in flight, sports
• 1/2000+ for motorsports

Intentional blur:

• 1/30 or slower for motion blur effects
• 1 second+ for flowing water
• 30 seconds+ for star trails

The Handheld Rule

For sharp handheld shots, use a shutter speed of at least 1/(focal length).

• 50mm lens → 1/60 minimum
• 100mm lens → 1/125 minimum
• 200mm lens → 1/250 minimum

This is a guideline, not a law. Steady hands can go slower; shaky conditions require faster.

ISO (Film Speed)

ISO measures film sensitivity to light. Unlike aperture and shutter speed, you can't change ISO mid-roll — it's fixed when you load the film.

Common ISO Values

Each doubling of ISO doubles sensitivity:

• ISO 400 needs half the light of ISO 200
• ISO 800 needs half the light of ISO 400

ISO and Grain

Higher ISO films have larger grain crystals. This creates visible texture in the image.

Low ISO (50-100):

• Very fine grain
• Smooth tones
• Maximum detail
• Needs plenty of light

Medium ISO (400):

• Moderate grain
• Good balance
• Most versatile

High ISO (800-3200):

• Visible grain
• Can add character
• Essential for low light
• Some detail loss

Grain isn't necessarily bad — it's an aesthetic choice. Many photographers love the look of Tri-X or HP5 at 1600 or 3200.

Choosing Your ISO

Since you're committed for the whole roll:

• Bright outdoor day: ISO 100-200
• Variable conditions: ISO 400 (the versatile choice)
• Indoor/low light: ISO 800
• Night/concerts: ISO 1600-3200

When in doubt, ISO 400 handles most situations. It's fast enough for shade but fine enough for enlargement.

Putting It Together

The exposure triangle means these three elements must balance. The same exposure can be achieved many ways:

Example: Sunny day, proper exposure

All these combinations give identical exposure:

• f/16, 1/125, ISO 100
• f/11, 1/250, ISO 100
• f/8, 1/500, ISO 100
• f/5.6, 1/1000, ISO 100
• f/4, 1/500, ISO 50

The exposure is equivalent, but the results differ:

• f/16 gives deep depth of field
• f/5.6 at 1/1000 freezes action with shallower depth
• f/4 with ISO 50 creates shallow depth and fine grain

Reciprocity in Practice

This interchangeability is called reciprocity — each stop of aperture can be traded for a stop of shutter speed (or ISO).

Want to shoot at f/2.8 instead of f/8? That's 3 stops more light through the lens. Compensate with 3 stops faster shutter speed: 1/60 becomes 1/500.

Making Choices

When setting exposure, ask:

1. What ISO am I working with? (Fixed by your film choice)
2. What depth of field do I want? (Set aperture accordingly)
3. What shutter speed does that require? (To balance the exposure)

Or reverse it:

1. What ISO am I working with?
2. Do I need to freeze/blur motion? (Set shutter speed)
3. What aperture does that require?

There's rarely one "correct" exposure — there's the exposure that achieves your creative intent.

Stops Explained

A "stop" is a doubling or halving of light. This universal unit lets you think about exposure changes consistently.

One stop more light:

• Open aperture one step (f/8 → f/5.6)
• OR slow shutter one step (1/250 → 1/125)
• OR use film one step faster (ISO 200 → ISO 400)

One stop less light:

• Close aperture one step (f/5.6 → f/8)
• OR speed up shutter one step (1/125 → 1/250)
• OR use film one step slower (ISO 400 → ISO 200)

When someone says "I needed to add two stops," they mean they needed four times as much light (2 × 2 = 4).

Exposure Compensation

Your camera's meter suggests settings to render the scene as middle grey. Sometimes you want to override this.

Add exposure (+1, +2 stops) for:

• Snow, white surfaces
• Backlit subjects
• High-key (bright) aesthetic

Subtract exposure (-1, -2 stops) for:

• Dark subjects against dark backgrounds
• Preserving shadow detail in contrasty light
• Low-key (dark) aesthetic

With negative film, slight overexposure is usually safer than underexposure. Shadow detail is easier to recover than blown highlights are to recreate.

Common Scenarios

Sunny Outdoor

Start with Sunny 16: f/16 at 1/ISO.

• ISO 400 film → f/16 at 1/500
• Want shallower depth? Open to f/5.6, use 1/4000 (if available) or accept some overexposure

Overcast Day

Two stops more than sunny (f/8 instead of f/16, or equivalent):

• ISO 400 film → f/8 at 1/500, or f/5.6 at 1/1000

Indoor with Window Light

Highly variable. Start around:

• ISO 400 film → f/2.8 at 1/60

Meter carefully; indoor light varies enormously.

Golden Hour

About one stop less than midday sun:

• ISO 400 film → f/11 at 1/500, or f/8 at 1/1000

Deep Shade

Three to four stops less than sun:

• ISO 400 film → f/4 at 1/125, or f/2.8 at 1/250

Summary

• Aperture controls light volume and depth of field. Smaller f-numbers = more light, shallower focus.
• Shutter speed controls light duration and motion. Slower speeds = more light, more blur.
• ISO is fixed per roll. Higher numbers = more sensitivity, more grain.
• Stops are doublings/halvings of light, the universal exposure unit.
• Trade one element for another to maintain exposure while changing creative effects.

Understanding exposure is understanding light. Once these concepts click, you'll read scenes instinctively — knowing before you meter roughly what settings you'll need, and choosing the combination that serves your vision.