Part 16: CALIBRATION

When you photograph a scene, you need a way to verify exposure is correct. The solution: 18% grey — a surface that reflects exactly 18% of incoming light.

Why 18%? It's the geometric mean of black (0%) and white (100%):

√(0 × 100) ≈ 18%

This represents the "middle" of a scene's luminance distribution.

Professional calibration uses standardized reflectance values:

When a camera's meter sees 18% grey and reports "correct exposure," everything in the scene will be properly exposed:

• Shadows retain detail
• Highlights don't clip
• Mid-tones appear natural

If 18% grey looks too dark → underexposed If 18% grey looks too bright → overexposed

In linear light (what physics measures), 18% is 0.18. In perceptual space (what we see), this appears as "middle grey" — about halfway between black and white.

Linear 0.18 ≈ sRGB 0.5 (middle grey)

This is why gamma correction exists — to map
physical values to perceptual values.

A calibration Blueprint gives you:

• Consistent reference across all scenes
• Quick verification of exposure settings
• Visual check of lighting quality
• Training data consistency (for ML workflows)

Create a Blueprint with four reference spheres:

Content Browser → Right-click → Blueprint Class → Actor
Name: BP_LHT_Calibration

Components

Material Setup

For the reference spheres, create simple unlit or base color materials:

Black Material:
  Base Color: (0.05, 0.05, 0.05)
  Roughness: 1.0

Grey Material:
  Base Color: (0.18, 0.18, 0.18)
  Roughness: 1.0

White Material:
  Base Color: (0.9, 0.9, 0.9)
  Roughness: 1.0

Chrome Material:
  Base Color: (1, 1, 1)
  Metallic: 1.0
  Roughness: 0.0

Note: These values are in linear space. If using sRGB input, the values would be different.

Blueprint Layout

Arrange the spheres in a row with clear separation:

┌─────────────────────────────────────────┐
│                                          │
│  ●        ●        ●        ◐           │
│ Black   Grey    White   Chrome          │
│  5%      18%      90%    Mirror         │
│                                          │
└─────────────────────────────────────────┘

1. Place BP_LHT_Calibration in your scene
2. Position it where your subject will be lit
3. Adjust exposure until the grey sphere looks "middle grey"
4. Verify: black has detail, white isn't clipped
5. Check chrome ball for reflection quality

UE5 has multiple places to control exposure. They layer on top of each other:

Project Settings (base default)
    ↓ overridden by
Viewport Settings (editor preview only)
    ↓ overridden by
Post Process Volume (scene-specific)
    ↓ overridden by
Camera Actor / Cine Camera (per-camera)
    ↓ separate context
Scene Capture (must be configured manually)

In both PPV and Cameras, each setting has a checkbox:

• Unchecked = Inherit from level above
• Checked = Override with this value

This lets you override only what you need.

The global defaults for your entire project:

Edit → Project Settings → Engine → Rendering → Default Settings

Apply Physical Camera Exposure

This toggle must be enabled for ISO and Shutter Speed to affect exposure.

Project Settings → Rendering → Default Settings:
  [x] Apply Physical Camera Exposure

Without this, ISO and Shutter Speed values are ignored — only Exposure Compensation matters.

When working in the editor, you might want to preview different exposure without changing your project:

Viewport → Lit dropdown → Exposure → [options]

Critical: Viewport exposure is editor-only. It does NOT affect Play mode, packaged builds, or cinematics.

If your scene looks right in the viewport but wrong in Play mode, check if you're using viewport exposure override.

Post Process Volumes control exposure for regions of your level:

1. Place Actors → Post Process Volume
2. Details → Post Process Volume Settings
3. Enable Infinite Extent (Unbound) for level-wide settings

The fundamental choice:

In Manual mode, exposure is entirely controlled by Exposure Compensation:

Exposure Multiplier = 2^(Exposure Compensation)

EC = 0   →  1×    (neutral)
EC = +1  →  2×    (twice as bright)
EC = -1  →  0.5×  (half as bright)
EC = +2  →  4×    (four times as bright)

In Manual mode, Min/Max EV100 are ignored. Only Exposure Compensation matters.

UE5 offers two auto exposure algorithms:

With Auto Basic or Auto Histogram:

Scene Brightness
    ↓
Calculate EV from scene
    ↓
Clamp to Min/Max EV100 range
    ↓
Apply Exposure Compensation offset
    ↓
Final Exposure

Different controls move in different directions. Reference this when confused:

EV100 describes what the scene IS:

• Higher = "scene is bright" → camera compensates darker
• Lower = "scene is dim" → camera compensates brighter

Exposure Compensation describes what you WANT:

• Positive = "make it brighter"
• Negative = "make it darker"

If you have overlapping volumes:

1. Enable Apply Physical Camera Exposure (Project Settings)
2. Place BP_LHT_Calibration in scene
3. Add Post Process Volume (Infinite Extent)
4. Set Metering Mode to Manual
5. Adjust Exposure Compensation until grey sphere looks correct
6. Verify: black has detail, white isn't clipped
7. Check chrome ball for lighting environment quality

Key Points:

• 18% grey is the universal mid-tone reference (linear 0.18)
• Calibration Blueprint with black (5%), grey (18%), white (90%), chrome spheres
• Exposure hierarchy: Project Settings → Viewport → PPV → Camera
• Apply Physical Camera Exposure must be enabled for ISO/Shutter to work
• Manual mode: Exposure Compensation controls everything
• Auto modes: Scene analysis + Min/Max EV100 clamping
• Exposure Compensation is intuitive (positive = brighter)
• EV100 is counterintuitive (higher = darker output)

Now that you understand exposure calibration, in Part 17: Camera Matching we'll configure UE5's camera systems to match real-world cameras — sensor sizes, focal lengths, and the complete matching workflow.