Part 15: CAMERAS

Whether you're using your eyes or a camera, light follows a similar path — though cameras add a step that eyes don't have:

Human eye anatomy showing light path from cornea through lens to retina

Your eye automatically adjusts:

• Pupil size for brightness (like aperture)
• Lens shape for focus distance
• Sensitivity over time (dark adaptation)

Camera anatomy showing light path from scene through lens, aperture, shutter to sensor

DSLR cutaway showing sensor behind the mirror

DSLR cameras have a mirror that reflects light to the viewfinder. The mirror flips up during exposure to let light hit the sensor.

Mirrorless camera cutaway showing sensor location

Mirrorless cameras remove the mirror — light goes straight to the sensor. Simpler, lighter, but no optical viewfinder.

Your eye receives light continuously — there's no shutter. Your brain processes a constant stream of visual information.

A camera, by contrast, captures discrete moments. The shutter opens, light hits the sensor for a specific duration, then the shutter closes. This is why cameras need explicit control over exposure time — something your eye handles automatically through continuous adaptation.

Focal length is the distance from the lens's optical center to the sensor when focused at infinity. It determines field of view — how much of the scene the camera sees.

Focal length is measured in millimeters (mm) — the physical distance from the lens's optical center to the sensor plane:

Focal length definition and FOV comparison

Shorter focal length = wider field of view Longer focal length = narrower field of view (more "zoom")

A 50mm prime lens is always 50mm. A 24-70mm zoom can be set anywhere in that range.

Focal length affects how objects appear relative to each other:

This is why portrait photographers use 85mm+ lenses — the compression is flattering. Wide lenses make noses look bigger and ears smaller.

Exposure is the total amount of light that reaches the sensor or film. Too little light = dark image (underexposed). Too much light = bright, washed out image (overexposed).

Correct exposure produces an image where:

• Shadows have detail (not pure black)
• Highlights have detail (not pure white)
• Mid-tones look natural

Your eye handles exposure automatically:

• Pupils dilate in darkness (more light in)
• Pupils contract in brightness (less light in)
• Retina adapts sensitivity over minutes (dark adaptation)

You never think about it — your brain just handles it.

Cameras require manual or automatic control of three settings:

These three controls form the exposure triangle. They work together to achieve correct exposure:

The exposure triangle - aperture, shutter speed, and ISO balance to create correct exposure

Change one, and you must compensate with another to maintain the same exposure. The next three sections explain each control in detail.

The aperture is an adjustable opening inside the lens that controls how much light passes through. Think of it like the pupil of your eye.

The aperture is made of overlapping blades that form a roughly circular opening. The number of blades affects the shape of out-of-focus highlights (bokeh).

Aperture is measured in f-stops. Counterintuitively, lower numbers = larger openings:

Aperture blades at f/2.8, f/8, and f/16 showing decreasing opening size

Why the confusing numbers? The f-stop is a ratio: focal length ÷ aperture diameter.

50mm lens at f/2 = 25mm opening (50 ÷ 2)
50mm lens at f/8 = 6.25mm opening (50 ÷ 8)

Each stop halves the light (or doubles it, going the other way):

f/1.4 → f/2 → f/2.8 → f/4 → f/5.6 → f/8 → f/11 → f/16 → f/22
      ←—————————————— less light ——————————————→

Variable aperture lenses are cheaper but more complex to use — zooming in reduces your maximum aperture.

Aperture controls depth of field (DOF) — how much of the scene is in acceptable focus.

Wide aperture = shallow DOF = creamy background blur (bokeh) Narrow aperture = deep DOF = everything sharp

The shutter is a mechanical curtain or blade that controls how long light hits the sensor. It's like blinking — open, let light in, close.

Focal plane shutter (left) and leaf shutter (right)

Shutter speed is expressed as fractions of a second:

Each stop doubles the light (or halves it):

1/1000 → 1/500 → 1/250 → 1/125 → 1/60 → 1/30 → 1/15 → 1/8
       ←————————————— more light ———————————————→

Shutter speed controls motion blur — how moving subjects appear.

Fast shutter = frozen motion = less light Slow shutter = motion blur = more light

ISO measures how much light the sensor or film needs to create a properly exposed image.

You may hear ISO called film speed:

• Slow film (ISO 100) = needs lots of light
• Fast film (ISO 800+) = needs less light

Think of it like filling a bucket:

• Low ISO = big bucket, takes more water (light) to fill
• High ISO = small bucket, fills quickly with less water

Silver halide crystal comparison

Film: Larger silver halide crystals = more sensitive = higher ISO. You're committed for the whole roll.

Digital: The sensor has fixed sensitivity. "Raising ISO" amplifies the electrical signal — like turning up the volume on a quiet recording.

Higher ISO comes with a cost:

Film grain comes from larger crystals being visible. Digital noise comes from amplifying the signal — you amplify the noise floor too.

ISO 100: Photons → Electrons → 1× amplification → Clean signal
ISO 800: Photons → Electrons → 8× amplification → Signal + 8× noise

This is why shadows are noisier than highlights — shadows have weaker signal, so the noise is more visible relative to it.

Sensor size is the physical dimensions of the light-capturing surface, measured in millimeters.

In film cameras, this was the film format. In digital cameras, it's the sensor dimensions. Same concept, different technology.

Full frame vs smartphone sensor size comparison

1. Affects FOV (with focal length)

Same lens, different sensor = different field of view:

50mm on Full Frame: 47° FOV
50mm on APS-C: 32° FOV (1.5× crop)
50mm on Micro 4/3: 25° FOV (2× crop)

2. Affects DOF characteristics

Larger sensors allow shallower depth of field at the same field of view:

Full Frame @ 50mm f/2.0 (47° FOV):
  → Shallow DOF, creamy background blur

Phone @ 6.9mm f/1.68 (same 47° FOV):
  → Deep DOF, almost everything in focus

This is why phone "portrait mode" uses software blur — the tiny sensor physically cannot produce shallow DOF.

3. Affects noise performance

Larger sensors collect more light per pixel = less noise in low light.

Crop factor describes how a sensor "crops" the image compared to full frame:

"Equivalent" means same field of view, NOT same depth of field or compression.

Now that you understand each control individually, let's see how they work together:

The goal is correct exposure — not too bright, not too dark.

Each control adds or removes light in stops:

• 1 stop = doubling or halving light
• +1 stop = 2× brighter
• -1 stop = 0.5× brighter (half)

Change one control → compensate with another → same exposure.

Starting point: f/8, 1/125s, ISO 100 (correctly exposed)

Want shallower DOF? Open aperture to f/4 (+2 stops light)
Compensate: Speed up shutter to 1/500s (-2 stops light)
Result: Same exposure, shallower DOF

All of these produce identical brightness:

Same brightness, but each combination produces different side effects.

Sunny outdoor portrait:

Lots of light available
Want blurry background → Wide aperture (f/2.8)
Compensate → Fast shutter (1/2000s)
Keep clean → Low ISO (100)

Indoor sports/kids:

Limited light
Need frozen motion → Fast shutter (1/500s)
Open up → Wide aperture (f/2.8)
Still not enough light → Raise ISO (800-1600)
Accept some noise

Waterfall long exposure:

Want motion blur → Slow shutter (1/4s)
Too much light → Narrow aperture (f/16)
Keep clean → Low ISO (100)
Still too bright? → Need ND filter (blocks light)

Think of exposure as a budget:

• You have a "correct exposure" amount to hit
• Three ways to get there
• Each way has a cost (side effect)
• You choose which costs you're willing to pay

A light meter measures scene brightness and tells you what exposure settings to use. It's how you know if a scene is bright or dark before taking a photo.

Light meters typically report values in:

• EV (Exposure Value) — a standardized brightness scale
• Lux — absolute light measurement

Incident metering is more accurate but requires a separate handheld meter. Reflected metering is what every camera uses internally.

Every camera has a built-in light meter that reads reflected light from the scene. It suggests exposure settings based on the assumption that the scene averages to 18% grey (middle brightness).

This works most of the time. It fails when:

• Scene is mostly white (camera underexposes)
• Scene is mostly black (camera overexposes)
• Strong backlight (camera exposes for background)

EV100 (Exposure Value at ISO 100) gives you a standardized brightness scale:

UE5 provides tools to measure scene brightness:

Visualize > EV100

• Shows exposure values across the scene
• Helps verify lighting is in correct range

Visualize > Luminance

• Shows absolute brightness in nits (cd/m²)
• Useful for HDR workflows

Pixel Inspector

• Read exact values at a specific pixel
• Window > Developer Tools > Pixel Inspector

Light meters help you:

1. Verify lighting is in correct range — Is your sunny outdoor scene actually EV 15?
2. Match real-world reference values — Ensure consistency with physical cameras
3. Debug exposure issues — Is the scene too bright/dark, or is the camera misconfigured?

A classic photographer's shortcut:

On a sunny day (EV 15), use f/16 and shutter speed = 1/ISO.

This gives you a baseline without a light meter. Adjust from there for different conditions.

Key Points:

• Human eyes and cameras follow the same light pipeline — focusing, aperture, recording
• Focal length determines field of view; sensor size affects the crop
• Aperture controls light amount and depth of field (lower f-stop = more light, shallower DOF)
• Shutter speed controls light duration and motion blur (slower = more light, more blur)
• ISO controls sensitivity/gain (higher = needs less light, but more noise)
• Sensor size affects FOV, DOF potential, and noise performance
• The exposure triangle is a balancing act — change one, compensate with another
• Light meters measure scene brightness in EV — use them to verify your lighting

Now that you understand how cameras work, in Part 16: Calibration we'll learn how to verify correct exposure using reference materials and configure UE5's exposure systems.