Analyzing Footage
Introduction​
Before you can reconstruct a scene, you must understand the device that captured it as best as you can. All visual evidence (photographs, video footage) is the product of specific optical and sensor configurations. This page details the technical parameters you need to try to identify and replicate to ensure your 3D camera matches the real-world source.
Image Dimension & Aspect Ratio​
Image dimension refers to the width and height of the rendered image in pixels. This determines both resolution and aspect ratio.
Common Aspect Ratios​
- 16:9 (1920×1080, 3840×2160): Standard for modern video, smartphones, and most security systems.
- 4:3 (1024×768, 1600×1200): Common in older CCTV systems and legacy digital cameras.
- 1:1 (1000×1000): Square format used in social media.
Sensor Size​
The sensor is the physical component that captures light. Its size fundamentally affects the field of view.
Common Sensor Sizes​
| Format | Dimensions | Crop Factor | Typical Use |
|---|---|---|---|
| Full-Frame | 36 × 24 mm | 1.0x | Pro DSLRs, Reference Standard |
| APS-C | ~23.5 × 15.6 mm | 1.5x | Mid-range DSLRs, Mirrorless |
| Micro 4/3 | 17.3 × 13 mm | 2.0x | Compact Mirrorless, Drones |
| Smartphone | ~5 × 4 mm | 5-7x | Mobile Phones, Action Cams |
| CCTV (1/3") | ~4.8 × 3.6 mm | 7.2x | Security Cameras |
Why It Matters: A 50mm lens on a Full-Frame camera looks "normal." The same 50mm lens on a camera with a smaller sensor size acts different.
Focal Length​
Focal length (measured in mm or degrees) determines the magnification (zoom) and angle of view.
Common Ranges​
- 14-24mm (Ultra-Wide): CCTV, Action Cams. Captures a huge area but distorts edges (barrel distortion). Objects appear further away.
- 24-35mm (Wide): Standard smartphone main camera. Broad view, moderate distortion.
- 35-50mm (Standard): Roughly matches human eye magnification. Natural perspective.
- 85mm+ (Telephoto): Portraits. Compresses distance, making background objects appear closer to the foreground.
The "Dolly Zoom" Effect​
Focal length changes spatial relationships.
- Wide Angle + Close Camera: Exaggerates depth. Background seems far away.
- Telephoto + Far Camera: Compresses depth. Background seems right behind the subject.
Important: You cannot match a photo just by moving the camera. You must match the focal length to get the spatial relationships right.
Aperture & Depth of Field (DOF)​
Maybe remove. Depth of Field is the range of distance that appears sharp.
The 4 Factors Controlling DOF​
- Aperture (f-stop): Lower number (f/1.8) = Blurry background. Higher number (f/22) = Everything sharp.
- Focal Length: Longer lenses (zoom) = Blurrier background.
- Distance: Closer to subject = Blurrier background.
Investigative Implications​
- Smartphones & CCTV: Have tiny sensors and wide lenses. This creates Deep DOF (everything is in focus).
- "Portrait Mode": Fake software blur. Do not trust it for optical analysis.
- Scale: If your 3D scene is the wrong scale (e.g., a 10-meter tall human), DOF calculations will be wrong. Model to real-world scale.
Motion Blur & Shutter Speed​
Motion blur reveals the shutter speed, which indicates lighting conditions.
- Standard Blur (1/48 sec): Smooth, cinematic motion. Typical of movies and high-end video.
- No Blur (1/1000 sec): Staccato, crisp frames. Typical of smartphones in bright sunlight or action cameras.
- Excessive Blur (1/10 sec): Streaky motion. Typical of CCTV in low light (night mode).
Lens Distortion​
Real lenses are not perfect; they bend light, especially at the edges. This in contrast to Blenders Camera, which is mathematically perfect, and distortion can be added as a post-process.
- Barrel Distortion: Straight lines bow outward. Common in wide-angle (CCTV, GoPro).
- Pincushion Distortion: Straight lines bow inward. Common in telephoto lenses.
Possible Workflow:
- Undistort: Use software to straighten the footage lines.
- Match: Match your perfect 3D camera to the straightened footage.
- Redistort: Apply the distortion back to your 3D render to match the original raw footage.
Common Investigative Scenarios​
Security Camera (CCTV)​
- Sensor: Tiny (1/3" or smaller).
- Lens: Very wide (2.8mm - 4mm).
- Distortion: Heavy barrel distortion.
- Position: High angle, looking down.
Smartphone​
- Sensor: Small.
- Lens: Wide (approx 24-28mm equivalent).
- Artifacts: Rolling shutter (jello effect), aggressive digital sharpening, variable frame rate.
Body Cam / Action Cam​
- Lens: Ultra-wide / Fisheye (120°+ FOV).
- Distortion: Extreme barrel distortion.
- Position: Chest or head height.
Summary Checklist​
When analyzing a new piece of footage, determine:
- Resolution: What are the pixel dimensions?
- Sensor Size: What device took this? (Look at EXIF data).
- Focal Length: Is it wide (distorted edges) or telephoto (flat perspective)?
- Distortion: Do straight architectural lines curve?
- Motion: Is it crisp (fast shutter) or blurry (slow shutter)?
You can use this data to configure your virtual camera before attempting to match the geometry.